METHOD FOR ANALYSING RECYCLED PLASTIC Field of the Invention The present invention relates to the method of estimating or quantifying the content of recycled plastic in a plastic product using fluorescent compounds, and related methods and products. Background There is a growing use of synthetic polymers in packaging and applications. This leads to concern for the disposal at the end of the life of a plastic product and is of both environmental and economic significance. Governments across the globe are publishing standards and regulations to reduce the level of pollution arising from unrecycled plastics and to incentivise businesses to use more recycled material for packaging. This includes imposing taxes which apply to plastic products containing less than a specified percentage of recycled plastic. To determine if a tax may be liable it is important to rapidly quantify the recycled plastic content in plastics. Currently, recycled plastic content may be determined by changes in gel permeation chromatography (GPC), rheology and differential scanning calorimetry (DSC) values, but these analytical techniques do not attempt to fully quantify the content. In addition, these systems are too complicated and subsequent results are produced too slowly for mass application. Therefore, there is a need for a fast quantification method with simple operation to provide a quantification of the recyclable plastic content in a plastic product. Fluorescence is the emission of light by a substance that has absorbed electromagnetic radiation. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. It is well known that fluorescent compounds may be used as markers or sensors for the detection of various chemical species and employed in the preparation of materials which become responsive to both mechanical and temperature stress. A fluorometer is a device which may be used to measure the intensity and wavelength distribution of the emission spectrum after applying radiation to excite the compound. 4,4’-Bis(2-benzoxazolyl)stilbene (BBS), also known as (Z)-fluorescent brightener 393, is a fluorescence compound which exhibits consistently high absolute fluorescence quantum yields of Φ(fl) ≥ 0.88. BBS currently has uses as an optical brightener or whitening agent for polymeric materials, in the preparation of poly(propylene) and as a viscosity-sensitive fluorophore for curing parameters in fabrication of UV- cured automotive organic protective coatings. When dissolved in a medium, such as a polymeric matrix, BBS may undergo aggregation to reveal specific forms, such as the presence of a dimer. The extent of the aggregation is dependent on the weight percentage or concentration of BBS and the corresponding change in emission spectrum may be monitored on a fluorometer. While BBS and other similar compounds have been used in fluorescence-based applications, they have yet to be used in a method for improving the technique of recycled plastic content quantification. IT 1399259 describes a thermal stress-sensitive material comprising a polymeric matrix and a luminescent dye dispersed in the polymeric matrix. DE 4430663 describes a process for increasing the viscosity and heat stability of nylon 6 during extrusion by coupling the chain extension and crosslinking processes in a single process stage. WO2018070933A1 describes a polymeric material, a fluorescent chemical sensor comprising the polymer and a method of detecting a volatile organic chemical or amino acid in either gas or solution phase. US20200024406A1 describes a porous polymeric material with a π-conjugated system, fluorescent chemical sensors comprising the polymer and a method of detecting a volatile organic chemical or metal ion in solution phase. US20070077596A1 describes a polymer with a trace amount of a luminescent moiety, a device that incorporates the polymeric compositions, methods of using the device to detect analytes and a method of preparing the device. US20200271517 describes a method for identifying and quantifying a material or mixture of materials, where the material or the mixture of materials comprises one or more components X identifiable by means of spectroscopic methods and/or with a hyperspectral camera. The method comprises the steps of A. generating one or more signals by excitation with a radiation source in the range of 280n1100 nm and recording thereof by a suitable spectrometer system, a hyperspectral camera or a photodiode, B. evaluating the signal(s) and/or hyperspectral image(s) obtained and assigning the signal(s) and/or hyperspectral image(s) to a component X, and subsequently assigning the identified component X to a material or a mixture of materials, C. quantitatively determining the material or mixture of materials. The quantitative determination of the materials is done by measuring the change in the intensity of the signal of compound X as the concentration of compound X changes. There remains a need for a rapid quantification method which determines the content of recycled plastic in a plastic product using a fluorescent compound with specific forms having concentration dependences. The present invention has been devised in light of the above considerations. Summary of the Invention The present inventors have devised an estimation and quantification method for determining the recycled plastic content in a plastic product using a fluorescent compound as a marker. This fluorescence-based marking allows for a known master-batch of recycled plastic to be tracked and monitored across all the stages of an integrated supply chain as the recycled plastic is processed and diluted with virgin plastic. Upon producing a final plastic product near the end of the supply chain, the change in fluorescence characteristics in the fluorescent compound can be monitored using a fluorometer by comparing the fluorescence characteristics of a sample of the plastic product with the fluorescence characteristics of the original pure recycled plastic marked with the fluorescent compound (hereinafter referred to as the “marked recycled plastic”). It will of course be apparent that the same measurement conditions used to obtain the fluorescence characteristics of the original pure recycled plastic marked with the fluorescent compound (the marked recycled plastic) are used to obtain the fluorescence characteristics of the final plastic product. As is already known, 4,4’-bis(2-benzoxazolyl)stilbene (BBS) possesses important fluorescence properties. It is also known that BBS can exist in two specific forms: either a molecularly disperse (isolated) form or an aggregated form. The proportion of each form in existence is dependent on the concentration of BBS in a medium, such as when dispersed in the polymeric matrix of the recycled plastic. In the aggregated form of BBS, dimers form from the alignment of dipoles and the presence of secondary interactions which become more favourable at higher BBS concentrations. This reduces the energy level of the first excited state of the aggregated dimer form relative to that of the molecularly disperse form according to molecular exciton coupling theory. Subsequent excitations require less energy, therefore increasing the emission wavelength which causes a bathochromic shift. Inspired by this, the present inventors use this concentration dependence of the specific forms of the fluorescent compound to quantify the recycled plastic content in a final plastic product. This can be done by measuring the ratio between the fluorescence intensity of a peak relating to the aggregated form to the fluorescence intensity of a peak relating to the isolated fluorescent compound in the emission spectrum of the final plastic product and comparing this against standardised or reference emission information, which may be a calibration curve, which has been prepared for the raw material recycled plastic containing the fluorescent compound. From this comparison, one can obtain a weight percentage (hereinafter referred to as weight % or wt %) of the fluorescent compound in the final plastic product. As the content of fluorescent compound in the raw material is known, the difference between the weight percentage of fluorescent compound in the final plastic product and the weight percentage of fluorescent compound in the recycled plastic correlates with the recycled plastic content and can thus be used to calculate that content. This new approach enables the use of a variety of fluorescent compounds, including but not limited to BBS, which exhibit similar fluorescence and concentration-dependent properties to BBS, as fluorescent markers for recycled plastic. The proportion of the aggregated form of such markers will change depending on their final concentration in the plastic product. As this concentration is proportional to the dilution of the recycled plastic with virgin plastic in a plastic product made from a mixture of the two plastic components, there now exists a method to quantify the recycled plastic content in a plastic product. It will be apparent that such quantification relies on the final plastic product being one which contains an amount of the fluorescent marker compound (that is, which has been made using at least some amount of a marked recycled plastic as discussed herein). It is also necessary that reference fluorescent emission information, and other characteristic information, spectra or values if used as described herein, is held for the raw material marked recycled plastic that is contained in the final plastic product. A notable, while not necessary, advantage of using BBS as a fluorescence compound in the present invention is that the compound is FDA approved. Commercially, this means that the method may be easily adopted in the quantification of recycled plastic content in food packaging and other food-related plastic products. Advantages of the method include that it is independent of sample dimensions and processing conditions, has little effect on polymer properties, and is inexpensive and highly compatible with existing recycling infrastructure. Discussed herein is a method for quantifying recycled plastic content in a plastic product. In a first aspect, the invention may provide a method of quantifying the recycled plastic content of a plastic product comprising a marked recycled plastic, the marked recycled plastic comprising recycled plastic and a known weight % of a fluorescent compound, and having reference fluorescent emission information; the method comprising the steps of: (a) obtaining a fluorescent emission spectrum of the plastic product to produce a plastic product fluorescent emission spectrum; (b) comparing the plastic product fluorescent emission spectrum to the reference fluorescent emission information and using that comparison to estimate the weight % of the fluorescent compound in the plastic product; (c) using the estimated weight % of the fluorescent compound in the plastic product from step (b), along with the known weight % of the fluorescent compound in the marked recycled plastic, to calculate an estimate of the recycled plastic content in the plastic product. Preferably, the plastic product comprises the marked recycled plastic and an unmarked virgin plastic. In some embodiments, the fluorescent compound self-associates by aggregation, and the isolated fluorescent compound has a first (isolated) emission spectrum and wherein aggregated fluorescent compound has a second (aggregated) emission spectrum, wherein at least one characteristic peak is observed at different wavelength in the first and second spectra, such that said characteristic peak is observed to shift from a first wavelength to second wavelength on aggregation of the fluorescent compound; wherein the reference fluorescent emission information is provided as a calibration curve obtained by taking a series of spectra for marked recycled plastics containing increasing weight % of the fluorescent compound and plotting the ratio of characteristic peak intensity at the first wavelength to intensity at the second wavelength; wherein step (b) comprises determining the characteristic peak ratio in the plastic product fluorescent emission spectrum and comparing that ratio to the calibration curve to obtain a reading corresponding to a known weight % of the fluorescent compound. In some embodiments the fluorescent compound has one of the following general formulae (a) to (h): wherein each of R ¹ to R ⁷⁰ , where present, is independently selected from hydrogen, halogen, nitro, cyano, alkoxy, and substituted or unsubstituted alkyl, wherein R ⁷¹ and R ⁷² , where present, are each independently selected from hydrogen, substituted alkyl and unsubstituted alkyl; and wherein R ⁷³ , where present, is selected from hydrogen, halogen, nitro, cyano, alkoxy, and substituted or unsubstituted alkyl. Herein, “substituted or unsubstituted alkyl” may suitably mean unsubstituted C _1-12 alkyl, or C _1-12 alkyl substituted with one or more groups selected from -F, -Cl, -Br, -OH, -OCH ₃ , -SH, -NH ₂ , -NHCH ₃ , -N(CH ₃ ) ₂ and -CO ₂ H. Both branched and straight chained alkyl groups are encompassed. It may suitably be a straight chained C _1-8 alkyl, C _1-5 alkyl, or C _1-3 alkyl, unsubstituted or substituted as mentioned above. Herein, “halogen” may suitably mean selected from –F, -Cl and –Br. Herein, “nitro” means –NO ₂ . Herein, “cyano” means –CN. Herein, “alkoxy” means –OR ^x , wherein R ^x is unsubstituted alkyl, for example straight chain unsubstituted C _1-8 alkyl, C _1-5 alkyl, or C _1-3 alkyl. In some embodiments, the fluorescent compound is of general formula (a), (b), (c) or (g). More preferably, the fluorescent compound is of general formula (a). In some embodiments, each of R ¹ to R ⁷⁰ , where present, is independently selected from hydrogen, -CH ₃ , R ⁷¹ and R ⁷² , where present, may preferably be hydrogen or alkyl, for example as defined above. R ⁷¹ may preferably be hydrogen or –C(CH ₃ ) ₃ . R ⁷² , where present, may preferably be –CH ₃ or - CH ₂ C(C ₂ H ₅ )CH ₂ CH ₂ CH ₂ CH ₃ . R ⁷³ , where present, may preferably be –CN. In some embodiments, where the fluorescent compound is of general formula (a), exactly one of R ¹ to R ⁵ , In some such embodiments, R ³ and R ⁸ are the same. For example, they may both be d t ^{1 10 71} he rest of R to R are H. In such embodiments R may preferably be H. In some embodiments, where the fluorescent compound is of general formula (b), R ¹¹ and R ¹⁴ are each are H. More preferably, R ¹¹ and R ¹⁴ are the same. For example, they may both be the group suc ⁷¹ h embodiments R may preferably be –C(CH ₃ ) ₃ . In some embodiments, where the fluorescent compound is of general formula (c), exactly two of R ¹⁵ to R ²⁰ are , the rest of R ¹⁵ and R ²⁰ are H or –OCH ₃ . Preferably, exactly two of R ¹⁵ to R ²⁰ are tly two of R ¹⁵ and R ²⁰ are also –OCH ₃ , and the rest of R ¹⁵ and R ²⁰ are H. It may be that R ¹⁵ and R ¹⁸ are ¹⁶ , R ¹⁷ , R ¹⁹ and R ²⁰ are H. Alternatively, R ¹⁶ and R ¹⁹ may be –OCH ₃ . In such embodiments, there is no requirement for each R ⁷² and each R ⁷³ in the fluorescent compound to be the same. That is, one R ⁷² may be different from the other R ⁷² , or they may be the same. One R ⁷³ may be different from the other R ⁷³ , or they may be the same. It may be preferred that each R ⁷² is the same and each R ⁷³ is the same. In some such embodiments, each R ⁷³ may preferably be –CN. Furthermore, in some such embodiments each R ⁷² may preferably be –CH ₃ or - CH ₂ C(C ₂ H ₅ )CH ₂ CH ₂ CH ₂ CH ₃ . Each R ⁷² may preferably be –CH ₃ where R ¹⁶ , R ¹⁷ , R ¹⁹ and R ²⁰ are H. Each R ⁷² may preferably be –CH ₃ or –CH ₂ C(C ₂ H ₅ )CH ₂ CH ₂ CH ₂ CH ₃ where R ¹⁶ and R ¹⁹ are –OCH ₃ . In some embodiments, where the fluorescent compound is of general formula (g), each of R ⁴⁹ to R ⁶⁰ is H. In other embodiments, where the fluorescent compound is of general formula (g), R ⁵⁰ and R ⁵¹ , and R ⁵⁶ and R ⁵⁷ , may be linked to form a 6- or 7- membered heterocyclic structure. The term heterocyclic used herein means a carbocyclic structure in which one of the ring carbon atoms is replaced with a heteroatom such as nitrogen (as, for example, NR ⁷⁶ ), oxygen (as O) or sulfur (as S). The atoms of the resultant cyclic structure may be optionally substituted, for example with a group R ⁷⁴ or R ⁷⁵ as set out below. For example, each of these pairs of substituents may form a group: wherein X is selected from NR ⁷⁶ , O, S, CH ₂ NR ⁷⁶ , CH ₂ O and CH ₂ S, and is preferably NR ⁷⁶ . R ⁷⁴ and R ⁷⁵ may be the same or different, and may preferably be independently selected from alkyl, alkoxy and oxo (that is,O linked to the carbon by a double bond). They may both be oxo. R ⁷⁶ may be alkyl as defined herein. This may lead to a structure: In some embodiments R ⁷⁷ , where present, is In some embodiments R ⁷⁸ , where present, is In some embodiments the fluorescent compound is 4,4’-bis(2-benzoxazolyl)stilbene (BBS), 1,4-bis(R- cyano-4-methoxystyryl)-2,5-dimethoxybenzene (BCMDB), 1,4-bis(R-cyano-4-methoxystyryl)benzene (BCMB), 1,4-bis(R-cyano-4-(2-ethylhexyloxystyryl))-2,5-dimethoxybenz ene (BCEDB), 2,5-Bis(5-tert-butyl- benzoxazol-2-yl)thiophene (BBT), 4-(2-Benzoxazolyl)-4'-(5-methyl-2-benzoxazolyl)stilbene (BMBS), 1,4- Bis(benzo[d]oxazol-2-yl)naphthalene (BBON), or 1,2-Bis(5-methyl-2-benzoxazolyl)ethylene (BMBE). In some embodiments the virgin plastic and / or the recycled plastic are selected from polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), polystyrene (PS) and polypropylene (PP). In some embodiments, the known weight % of the fluorescent compound in the marked recycled plastic is 1 wt% or less with respect to the weight of the recycled plastic. In some embodiments the method may be one wherein the marked recycled plastic has reference fluorescent lifetime and resulting decay information, the method comprising the further steps: (a ¹ ) obtaining a fluorescent lifetime and resulting decay function of the plastic product, to produce a plastic product fluorescent lifetime and resulting decay function; and (b ¹ ) comparing the plastic product fluorescent lifetime and resulting decay function to the reference fluorescent lifetime and resulting decay information, to estimate the weight % of the fluorescent compound in the plastic product; (c ¹ ) using the weight % of the fluorescent compound in the plastic product estimated in step (b ¹ ), along with the known weight % of the fluorescent compound in the marked recycled plastic, to calculate an estimate of the recycled plastic content in the plastic product; (d ¹ ) combining the estimates made in steps (c) and (c ¹ ) to form a refined estimate of the recycled plastic content in the plastic product. In some embodiments the method may be one wherein the marked recycled plastic has reference optical Red, Green, Blue and Hue information obtained using Red, Green, Blue channels, the method comprising the further steps: (a ² ) obtaining optical Red, Green, Blue and Hue values of the plastic product using Red, Green, Blue channels; and (b ² ) comparing the optical Red, Green, Blue and Hue values of the plastic product to the reference optical Red, Green, Blue and Hue information to estimate the weight % of the fluorescent compound in the plastic product; (c ² ) using the weight % of the fluorescent compound in the plastic product estimated in step (b ² ), along with the known weight % of the fluorescent compound in the marked recycled plastic, to calculate an estimate of the recycled plastic content in the plastic product; (d ² ) combining the estimates made in steps (c), (c ¹ ) if present and (c ² ) to form a refined estimate of the recycled plastic content in the plastic product. Instead of Red, Green, Blue and Hue values, alternative colour spaces may be used, such as the L*a*b* colour space. Therefore, a split and comparison of values in the a* and b* channels can be useful. Accordingly, in some embodiments the method may be one wherein the marked recycled plastic has reference optical L*, a* and b* information obtained using L*a*b* colour space channels, the method comprising the further steps: (a ³ ) obtaining optical L*, a* and b* values of the plastic product using L*, a* and b* channels; and (b ³ ) comparing the optical L*, a* and b* values of the plastic product to the reference optical L*, a* and b* information to estimate the weight % of the fluorescent compound in the plastic product; (c ³ ) using the weight % of the fluorescent compound in the plastic product estimated in step (b ³ ), along with the known weight % of the fluorescent compound in the marked recycled plastic, to calculate an estimate of the recycled plastic content in the plastic product; (d ³ ) combining the estimates made in steps (c), (c ¹ ) if present, (c ² ) if present and (c ³ ) to form a refined estimate of the recycled plastic content in the plastic product. Any of the reference fluorescent emission information, reference fluorescent lifetime and resulting decay information, reference optical red, green, blue and hue information, or reference optical L*, a* and b* information may preferably be a calibration curve, line or table (for simplicity, herein “calibration curve” refers to any line, including straight line: the extent of curvature is unimportant). For example, in the case of the reference fluorescent emission information, it may be a reference fluorescent emission calibration curve which can be prepared by measuring fluorescent emission spectra for a series of different concentrations of the fluorescent compound within the marked recycled plastic; for each such concentration, for example, a ratio between a peak attributed to the aggregated form of the fluorescent compound and a peak attributed to the isolated form of the fluorescent compound can be measured. Then, these ratios can be plotted against the concentration used. To these data can be fitted a curve or line (using a known method such as fitting to a function [ratio] = a*ln([wt% fluorescent compound]) + c1; or a function [ratio] = a*([wt% fluorescent compound]) + c2). This calibration curve can then be used to read off values of the wt% fluorescent compound (x axis) for a plastic product from the y-axis value (peak ratio etc) measured. Analogous processes can be used to form calibration curves for the reference fluorescent lifetime and resulting decay information, reference optical red, green, blue and hue information, or reference optical L*, a* and b* information. Alternatively, the reference information may simply be a library of fluorescent emission spectra, reference fluorescent lifetime and resulting decay values, reference optical red, green, blue and hue values, or reference optical L*, a* and b* values taken for a series of different concentrations of the fluorescent compound within the marked recycled plastic. The measured value(s) or spectrum from the plastic product can be compared to this library to find the closest matching concentration of fluorescent compound in the reference information; this then serves as the estimated content in the plastic product. Another aspect of the present invention provides a method of marking a recycled plastic, comprising the steps of (a) melting the recycled plastic; and then (b) mixing a fluorescent compound as a marker with the melted recycled plastic in a pre-determined weight % to produce a marked recycled plastic. In some embodiments, such a method may further comprise the step, after step (b), of: (c) solidifying the marked recycled plastic. The method may also further comprise the step, after step (b) or step (c) if present, of: (d) obtaining a fluorescent emission spectrum of the marked recycled plastic to produce a reference fluorescent emission spectrum. The present invention also provides a marked recycled plastic comprising recycled plastic and a fluorescent compound. The present invention also provides a method of manufacturing a plastic product, comprising the steps of (a) compounding an unmarked virgin plastic with a marked recycled plastic as described herein, to make a marked diluted plastic; and (b) producing a plastic product from the marked diluted plastic. The present invention also provides a plastic product comprising a marked recycled plastic as described herein and unmarked virgin plastic. The present invention also relates to use of a fluorescent compound as a marker for recycled plastics, the fluorescent compound being as described herein. In a second aspect, the invention may provide a method of quantifying the recycled plastic content of a plastic product comprising a marked recycled plastic and unmarked virgin plastic, the marked recycled plastic comprising recycled plastic and a known weight % of a fluorescent compound, and having reference fluorescent emission information; the method comprising the steps of: (a) obtaining a fluorescent emission spectrum of the plastic product to produce a plastic product fluorescent emission spectrum; (b) comparing the plastic product fluorescent emission spectrum to the reference fluorescent emission information and using that comparison to estimate the weight % of the fluorescent compound in the plastic product; (c) using the estimated weight % of the fluorescent compound in the plastic product from step (b), along with the known weight % of the fluorescent compound in the marked recycled plastic, to calculate an estimate of the recycled plastic content in the plastic product. The method according to the second aspect may include method steps or features corresponding to method steps or features of the method according to the first aspect. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 is a schematic flow chart showing in general terms the methods described herein. Figure 2 shows Top) the emission spectra of BBS-PET samples excited at 325 nm. Bottom) The emission spectra of BBS-PET samples excited at 325 nm after annealing for 16 hours at 120°C. Figure 3 shows the emission spectrum of BBS-HDPE samples excited at 325 nm. Figure 4 shows a plot of BBS wt% against the ratio of fluorescent intensities at 470 nm and 430 nm (o) for BBS-HDPE (calibration curve). Plot of logarithmic data. Figure 5 shows a plot of BBS wt% against the squared ratio of fluorescent intensities at 470 nm and 430 nm (o) for BBS-HDPE (calibration line). Plot of linear data. Figure 6 shows Top) fluorescent lifetimes of BBS-PET samples excited at 340 nm and recorded at 500 nm (o) and bi-exponential fit. Bottom) Weighted residuals. Figure 7 shows images of BBS-HDPE samples at various virgin plastic:recycled plastic ratios. Figure 8 shows RGB channels of the BBS-HDPE samples of Figure 7 illuminated at 365nm. In particular, there can be seen a clear increase of green value with aggregation and bathochromic shift. The blue and red values stay constant. Figure 9 shows L*a*b* channels of the BBS-HDPE samples of Figure 7 illuminated at 365nm. In particular, there can be seen a clear increase of b* value and decrease of a* value with aggregation and bathochromic shift. Figure 10 shows a greyscale image of the BBS-HDPE samples of Figure 7 illuminated at 365nm at increased brightness. Figure 11 shows aggregation behaviour of 4-(2-Benzoxazolyl)-4'-(5-methyl-2-benzoxazolyl)stilbene (BMBS). Figure 12 shows aggregation behaviour of 1,4-Bis(benzo[d]oxazol-2-yl)naphthalene (BBON). Figure 13 shows aggregation behaviour of 1,2-Bis(5-methyl-2-benzoxazolyl)ethylene (BMBE). Figure 14 shows fluorescent emission spectra of varying recycled content for diluted for diluted 0.1 wt% BBS-HDPE master batch (“MB”). Figure 15 shows fluorescent lifetime measurements for diluted 0.1 wt% BBS-HDPE master batch (“MB”). Figure 16 shows colour analysis of 0.1 wt% BBS-HDPE MB samples. Figure 17 shows intensity data. Figure 18 shows fluorescent emission spectra of varying recycled content for PP recyclate marked by BBS, normalised to 1 at the fluorescent emission maxima of isolated molecules (left) and resulting intensity ratios between 470, 500 and 430 nm for PP recycled simulations. Figure 19 shows fluorescent emission spectra of varying recycled content for PET recyclate marked by BBS (0.5 wt%), normalised to 1 at the fluorescent emission maxima of isolated molecules (left) and resulting intensity ratios between 470, 500 and 430 nm for PET recycled simulations (right). Figure 20 shows fluorescent emission spectra of varying recycled content for annealed PET recyclate marked by BBS (0.5 wt%), normalised to 1 at the fluorescent emission maxima of isolated molecules (left) and resulting intensity ratios between 470, 500 and 430 nm for PP recycled simulations (right). Figure 21 shows fluorescent lifetime measurements for diluted 0.1 wt% BBS-PP MB. Figure 22 shows fluorescent lifetime measurements for diluted 0.5 wt% BBS-PET MB. Figure 23 shows fluorescent lifetime measurements for diluted annealed 0.5 wt% BBS-PET MB. Figure 24 shows colour analysis of 0.1 wt% BBS-PP MB samples illuminated at 365 nm and photographed in a blacked-out room. Figure 25 shows colour analysis of annealed 0.5 wt% BBS-PET MB samples illuminated at 365 nm and photographed in a blacked-out room. Figure 26 shows fluorescent emission spectra and resulting intensity ratios 0.1 wt% BBS-HDPE MB coloured samples. Figure 27 shows colour analysis of 0.1 wt% BBS-HDPE MB coloured samples illuminated at 365 nm and photographed in a blacked-out room. Detailed Description of the Invention Definitions of plastic components Recycled plastic The invention quantifies the content of recycled plastic. The term recycled plastic refers to the pure recycled plastic material. The recycled plastic may be, but not necessarily limited to, polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), polystyrene (PS) and polypropylene (PP). Preferably, the recycled plastic is a material which is more hydrophobic than poly(vinyl alcohol) (PVA). The recycled plastic may be a mixture of different polymers. It is understood in the art that recycled plastic means plastic that has been made into or used in a final (for example commercial) product at least once. Often recycled plastic is in the form of pellets made by destruction and reformation of existing plastic products into the recycled plastic pellets. There is no limitation to the number of times a plastic may have been recycled outside of the practical limitations of mechanical recycling before being used in the present invention. Marked recycled plastic As used herein, the marked recycled plastic is the master-batch material produced after the fluorescent compound is added to the recycled plastic. The fluorescent compound is at its highest concentration throughout the method in the marked recycled plastic. The dilution of the marked recycled plastic with virgin plastic, as discussed herein, lowers the overall concentration of the fluorescent compound in the plastic. Virgin plastic As used herein, the virgin plastic may refer to a non-recycled plastic derived from a resin which comes directly from a petrochemical feedstock. The virgin plastic does not contain any fluorescent compound – it is unmarked. The virgin plastic may be, but not necessarily limited to, polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), polystyrene (PS) and polypropylene (PP). Preferably, the virgin plastic is a material which is more hydrophobic than poly(vinyl alcohol) (PVA). The virgin plastic may be a mixture of different polymers. Marked diluted plastic As used herein, the marked diluted plastic is the material produced after the marked recycled plastic is compounded (mixed, or otherwise combined) with virgin plastic. In the marked diluted plastic, the fluorescent compound is at a lower concentration than when in the marked recycled plastic – it is diluted with virgin plastic. Plastic product As used herein, the plastic product or final plastic product refers to any product which comprises the marked diluted plastic. In the plastic product, the fluorescent compound is at the same concentration as it is in the marked diluted plastic. Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. A schematic drawing of the steps involved in methods described herein is provided in Figure 1. At their broadest, those steps include mixing a recycled plastic with a fluorescent compound to make a marked recycled plastic; and mixing that with virgin plastic to make a marked diluted plastic. That marked diluted plastic can then be made into a plastic product. To analyse the recycled plastic content of the plastic product, a sample of it is taken. By use of a fluorometer a fluorescent emission spectrum for the sample is obtained. That can be compared to known (reference) fluorescent emission information that had previously been obtained or generated for the marked recycled plastic. The comparison allows calculation of an estimated content of marked recycled plastic (and hence recycled plastic) in the plastic product. The present invention uses fluorescent compounds, also known as fluorophores. Suitable fluorescent compounds are molecules comprised of multiple combined or linked aromatic groups, or planar or cyclic molecules with multiple π bonds. For example, stilbene-based fluorescent compounds are known in the art. In the present invention, a fluorescent compound is added to a recycled plastic in a pre-determined (that is, known) weight %. Preferably, the fluorescent compound may be added to melt phase. It will be appreciated that the fluorescent compound is dispersed in the polymeric matrix of the recycled plastic in a uniform distribution. Thus the marked recycled plastic contains a uniform dispersion of the fluorescent compound. Then compounded/combined with virgin plastic to make a marked diluted plastic, that marked diluted plastic then contains a uniform dispersion of the fluorescent compound. This means that analysis of a sample of the plastic product (made from the marked diluted plastic) is representative of the plastic product as a whole in terms of the fluorescent compound and hence marked recycled plastic content. The fluorescent compound is not particularly limited; it is one which exhibits identifiable peaks in fluorescent emission spectra corresponding to a molecularly disperse form and an aggregated form, and which thereby displays concentration dependent fluorescence characteristics. The (for example, bathochromic) shift of peaks between the molecularly disperse (isolated) form and aggregated form permits a ratio of aggregated and isolated forms to be calculated, and thereby a concentration of the fluorescent compound. That is, it may preferably be a compound which self-associates by aggregation, wherein the isolated fluorescent compound has a first ‘isolated’ emission spectrum and wherein aggregated fluorescent compound has a second ‘aggregated’ emission spectrum, wherein at least one characteristic peak is observed at different wavelength in the first and second spectra, such that said characteristic peak is observed to shift from a first wavelength to second wavelength on aggregation of the fluorescent compound. In other words, the fluorescent compound may be one existing in a first form (such as the above molecularly dispersed or isolated form) and a second form (such as the above aggregated form), the first and second forms having different fluorescent emission spectra, and the relative population of the first and second forms being dependent on the concentration of the fluorescent compound. Preferably the fluorescent compound is stilbene-based. In a more preferred embodiment, the fluorescent compound is 4,4’-bis(2-benzoxazolyl)stilbene (BBS). It is generally expected that a stilbene-derived fluorescent compound would exhibit the best fluorescence and aggregation characteristics when observing its emission spectrum. The fluorescent compound must be capable of forming a secondary interaction, most preferably, although not limited to, π–π stacking, with itself, at particular weight % in the polymeric matrix of the recycled plastic. The form of the fluorescent compound must be concentration dependent when dispersed in the recycled plastic. Its concentration will determine whether the compound is present in a molecularly disperse form or an aggregated form. A greater concentration of the compound will shift the equilibrium towards the aggregated form. The aggregated form of the compound may for example be a dimer (or excimer), which is formed upon alignment of the dipoles of the molecules as they form the secondary interaction. According to molecular exciton coupling theory, the first excited state of the dimer is of a lower energy than the first excited state of the molecularly disperse (monomer) form. Subsequent excitations require less energy and this increases the wavelength of light upon emission. As mentioned above, the fluorescent compound is added at a pre-determined (known) weight % to the recycled plastic. A lower content may be preferred for cost and processing reasons. Suitably, the fluorescent compound may be added at a weight % of 1 wt% or less with respect to the weight of the marked recycled plastic [for example, 99kg of recycled plastic + 1kg fluorescent compound to make 100kg of marked recycled plastic would be a weight % of 1 wt% of the fluorescent compound with respect to the weight of the marked recycled plastic.] While a weight % of 1 wt% or less is suitable, the known additive amount may be, for example, 0.8 wt% or less, 0.6 wt% or less, 0.4 wt% or less, 0.2 wt% or less, or 0.1 wt% or less. On the other hand, the content must be sufficient to allow aggregation and hence a concentration dependent fluorescent response. The exact threshold for aggregation may depend on the identity of the host polymer. The factors which affect the threshold for aggregation may be dependent on the solubility of the fluorescent compound in the polymer and competing secondary interactions. As an example, the fluorescent compound may be used at a weight % of 0.05 wt% or more with respect to the recycled plastic. A higher content may be used to improve sensitivity or clarity of fluorescent response. There is no particular upper limit in the weight % of the fluorescent compound. However, at significantly high weight %, there may also be a need for greater dilution with virgin plastic in order to obtain an observable change in the emission spectrum. The recycled plastic may suitably be, but not necessarily limited to, selected from polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), polystyrene (PS) and polypropylene (PP); or it may be a mixture of two or more of these. Preferably, the recycled plastic is a material which is more hydrophobic than poly(vinyl alcohol) (PVA). The fluorescent compound is generally hydrophobic in nature to facilitate its dispersion in the polymeric matrix of the recycled plastic. A recycled plastic which is as hydrophobic or less hydrophobic than poly(vinyl alcohol) (PVA) may interrupt the formation of secondary interactions between the fluorescent compound. Additionally, there may be some disruption in the π–π stacking of the aggregated form with the phenyl groups of polyethylene terephthalate (PET) and polystyrene (PS). Use of these recycled plastics with the fluorescent compound may result in a greater signal-to-noise in the emission spectrum from a potentially increased threshold for aggregation which may limit the range of quantification. Once the marked recycled plastic is obtained, a marked recycled plastic sample can be taken from the marked recycled plastic for fluorescence analysis. There is no particular requirement for the marked recycled plastic sample size, shape or thickness. The marked recycled plastic sample is placed within a fluorometer. Electromagnetic radiation of known wavelength is radiated on the marked recycled plastic sample. The emission spectrum of the marked recycled plastic is recorded to produce a standardised or reference emission spectrum using the fluorometer. This or multiple such spectra can be used to generate reference fluorescent emission information. Preferably, the sample taken from the marked recycled plastic is a solid. If the sample is dissolved, the fluorescent compound may exhibit molecularly disperse emission behaviour because the diffusion time may become much shorter than the fluorescent lifetime. The reference fluorescent emission information may be a calibration curve which may be used for estimating the recycled plastic content in the plastic product. The calibration curve may be produced from the fluorescent intensity ratios of a series of variable known weight % of fluorescent compound in marked recycled plastic. For example, the reference fluorescent emission information may be in the form of a calibration curve, the calibration curve being obtained from a plurality of reference fluorescent emission spectra for marked recycled plastics containing different weight % of the fluorescent compound; the calibration curve having one axis corresponding to the weight % of the fluorescent compound in the given marked recycled plastic and the other axis corresponding to the fluorescent intensity ratio between a peak attributed to the first form of the fluorescent compound and a peak attributed to the second form of the fluorescent compound, In other words, the reference fluorescent emission information may be provided as a calibration curve obtained by taking a series of spectra for marked recycled plastics containing increasing weight % of the fluorescent compound and plotting the ratio of characteristic peak intensity at the first wavelength to intensity at the second wavelength; wherein step (b) comprises determining the characteristic peak ratio in the plastic product fluorescent emission spectrum and comparing that ratio to the calibration curve to obtain a reading corresponding to a known weight % of the fluorescent compound. The fluorescent intensity ratio is defined as the ratio between the peak intensity of a predetermined peak of the aggregated form and the peak intensity of a predetermined peak of the molecularly disperse form of the fluorescent compound and may be obtained from an emission spectrum of the marked recycled plastic. The emission spectrum may be normalised to the peak of the molecularly disperse form. The fluorescent intensity ratio may be generally represented by technical field, it is expected that the relevant peaks of aggregated and molecularly disperse forms can be identified by visual assessment of the fluorescent emission spectrum. According to molecular exciton coupling theory, the peak of the aggregated form is found at a longer wavelength than the peak of the molecularly disperse form on an emission spectrum. The wavelengths correspond to excitations and relaxations of electrons in the fluorescent compound. The aggregated form has a smaller energy gap which results in a longer wavelength. A bathochromic shift is therefore attributed to an increase in the proportion of the aggregated form. An example of an emission spectrum for PET including various levels of fluorescent compound (here, BBS) is shown in Figure 2, Top. A similar example of fluorescent emission spectra for HDPE including various levels of fluorescent compounds (here, BBS) is shown in Figure 3. For example, if BBS is used as the fluorescent compound, the highest intensity peak of the aggregated form may in some embodiments (such as those where the recycled plastic is HDPE) be at a wavelength of around 470 nm and the highest intensity peak of the molecularly disperse form may be at a wavelength of around 430 nm. In this particular case, the fluorescent intensity ratio may then be represented by There may be a further peak caused by the aggregated form at around 500 nm, which may increase in intensity with increased concentration of the fluorescent compound. In this particular case, the fluorescent intensity ratio may then be represented by . It should be noted that an emission spectrum may contain some quantity of both the molecularly disperse form and the aggregated form, and therefore the intensity of the peaks of the emission spectra may represent an average of the population of the two separate forms.470 nm and 430 nm may be the wavelengths of the highest intensity peaks for a “pure” aggregated form and a “pure” molecularly disperse form respectively, and the difference may be the bathochromic shift (around 40 nm). Herein, “isolated form” or “molecularly disperse form” refers to a wavelength maxima characteristic of the isolated or molecularly disperse form, and “aggregated form” refers to a wavelength maxima characteristic of the aggregated form. In a non-normalised emission spectrum, the high intensity peak at around 470 nm may correspond mainly to the aggregated form, but may contain some fluorescent contribution from the molecularly disperse form. Similarly, a high intensity peak at around 430 nm may correspond mainly to the molecularly disperse form, but may also contain some fluorescent contribution from the aggregated form. Preferably, by normalising the emission spectrum to the peak of the molecularly disperse form (around 430 nm), it can be assumed that any subsequent change in the intensity of the peak around 470 nm is attributed only to the change in the proportion of the aggregated form. It will be appreciated that there may be small variations in peak wavelengths as this is dependent on the host polymer which affects interactions with the fluorescent compound. In some embodiments, the variation of values discussed herein may be ± 15 nm, for example ± 5 nm or ± 3 nm. For example, in Figure 2, the relevant peaks in question corresponding to BBS at 1.675 wt% in marked PET are at wavelengths of 418nm, 442 nm and 469 nm, with a shoulder at 500 nm. The peaks at 418 nm, 442 nm and 469 nm correspond to the 0 → 0, 0 → 1 and 0 → 2 radiative transitions, respectively. Therefore, it will also be appreciated that multiple peaks corresponding to a given form of the fluorescent compound may be observed. Multiple peaks correspond to different radiative transitions. For the purposes of generating fluorescent intensity ratios, it is most preferable to compare the highest intensity peak corresponding to the isolated form with the peak or peaks corresponding to the aggregated form. In the case of the example above, the highest intensity peak corresponding to the isolated form is 442 nm (in the language above, around 430 nm), corresponding to the 0 → 1 transition (Figure 2). It should be noted that, while the 418 nm peak also corresponds to the molecularly disperse form, it is preferably not used for generating fluorescent intensity ratios due to its inherently relatively low intensity. Multiple peaks corresponding to the aggregated form may be used to generate fluorescent intensity ratio(s) independently and/or both may be used as a secondary check for consistency of data. In such cases, comparison of the peak around 430 nm (isolated form) with that around 470 nm (aggregated form) may be done; alternatively, comparison of the peak around 430 nm (isolated form) with that around 500 nm (aggregated form) may be done. Both these may be done, and the resulting estimates of recycled plastic content may be combined to obtain a refined estimate. Those skilled in this technical field will appreciate that, in the fluorescent emission spectrum, the lower wavelength peak(s) may correspond to the molecularly disperse form of the fluorescent compound and higher wavelength peak(s) may correspond to the aggregated form(s), somewhat irrespective of the exact wavelengths at which those peaks lie. In the case of multiple peaks corresponding to the molecularly disperse form, it is preferable to use the highest intensity peak. It will also be appreciated that in the present invention the fluorescent compound to be added is pre- selected, that is, the operator of the method knows what that compound is and will be. Therefore, its spectrum can be obtained and analysed before addition, in order to identify the wavelength of the most suitable peak for analysis. As long as the ‘same’ peaks are used consistently (that is, intensity of the same wavelength peak corresponding to the molecularly disperse compound is used in comparison to the intensity of same wavelength peak corresponding to the aggregated compound across different spectral analyses) the required calculations can be completed, for example in generation of a calibration curve and reading from that for a plastic product of unknown marked recycled plastic content. The sample may optionally be quenched post-extrusion and annealed, for example for 16 hours at 120°C, to increase bathochromic shift. Annealing increases the polymer crystallinity which increases the number of fluorescent compound molecules in the aggregated form. This may be preferable for polymers with high amorphous fractions, such as PET, where a “shoulder” in the emission spectrum may be observed (Figure 2, Bottom). This change only occurs where the weight percentage is above the minimum weight percentage required for aggregation. BBS at 1.675 wt% in marked PET significantly changes the emission spectrum (Figure 2, Bottom), but there may be little effect on the emission spectrum when the weight percentage is lower than 1%. Accordingly, if a significant change in fluorescent response of a plastic product is observed, the skilled person may compare such a response against the emission spectrum of a marked recycled plastic in which the fluorescence compound is at 1 wt% or less to confirm that said plastic product has been at least annealed to alter its emission spectrum and consequently its perceived fluorescence intensity ratio and perceived recycled plastic content. Each value of fluorescence intensity ratio be plotted against its corresponding known weight % of the fluorescent compound in the marked recycled plastic to produce a calibration curve having a logarithmic relationship of the form (see,for example, Figure 4). Preferably, each value of may be plotted against its corresponding weight % of the fluorescent compound in the marked recycled plastic to produce a calibration curve or line having a linear relationship of the form (see, for example, Figure 5). X may be one of several values. For example, X may be 1 or 2. A suitable range of variable known weight % of the fluorescent compound for producing the calibration curve or line may be from 0.005 wt% to 1.675 wt%. That is, values of the fluorescence intensity ratio, or its square, can be measured and found for a series of samples having weight % of the fluorescent compound within this range, for example 0.025 wt%, 0.05 wt%, 0.125 wt%, 0.1675 wt%, 0.25 wt%, 0.5 wt%, 1.25 wt% and 1.675 wt%. It will be appreciated that not all of these must be measured to give a useful calibration curve: for example, using 3, 4 or 5 values within the range 0.005 wt% to 1.675 wt% may be sufficient. Ideally the values used are spaced apart, for example for that there is no less than 0.025 wt% between values. The data obtained, and the thus calculated calibration curve or line, may be computationally extrapolated to increase the range of calibration of the weight % in either direction if necessary, for example, down to 0 wt% and/or over 1.675 wt%. Once the plastic product fluorescent emission spectrum is obtained from the plastic product, the fluorescence intensity ratio of the plastic product is determined from the plastic product fluorescent emission spectrum by calculating the ratio between the peak intensity of the aggregated form and the peak intensity of the molecularly disperse form. The fluorescence intensity ratio of plastic product is then compared to the reference fluorescent emission information, for example ‘plotted’ onto the calibration curve. The weight % of the fluorescent compound in the plastic product can then be determined by reading off the weight % axis of the calibration curve. The weight % of the fluorescent compound in the plastic product is then divided by the known weight % of the fluorescent compound in the marked recycled plastic which was known to have been used to make the plastic product to calculate an estimate of the recycled plastic content in the plastic product according to the following equation: In another embodiment, fluorescent lifetime decay function may be used in a similar way. For example, a reference fluorescent lifetime and resulting decay function (corresponding to reference fluorescent lifetime and resulting decay information) may be produced from the fluorescent lifetimes of a series of variable known weight % of fluorescent compound in marked recycled plastic. The various known weight % may be as described above. The plastic product fluorescent lifetime and resulting decay function is obtained from the plastic product (see, for example, Figure 6). The plastic product fluorescent lifetime and resulting decay function is compared against the reference fluorescent lifetime and resulting decay function, and the changes in the resulting decay functions are observed to obtain an estimate for the weight % of the fluorescent compound in the plastic product. The weight % of the fluorescent compound in the plastic product is then divided by the known weight % of the fluorescent compound in the marked recycled plastic which was known to have been used to make the plastic product to calculate an estimate of the recycled plastic content in the plastic product according to the following equation: The estimated value of recycled plastic content obtained from the plastic product fluorescent lifetime and resulting decay function may be combined with the estimated value of recycled plastic content from the plastic product fluorescent emission spectrum to form a refined estimate of the recycled plastic content in the plastic product. In another embodiment, reference optical Red, Green, Blue and Hue values obtained using Red, Green and Blue channels may be used in a similar way. For example, reference optical Red, Green, Blue and Hue values may be produced from the optical Red, Green, Blue and Hue values of a series of variable known weight % of fluorescent compound in marked recycled plastic. The various known weight % may be as described above. The optical Red, Green, Blue and Hue values are obtained using Red, Green and Blue channels from the plastic product (see, for example, Figures 7 to 10). The optical Red, Green, Blue and Hue values of the plastic product are compared against the reference optical Red, Green, Blue and Hue values to obtain an estimate for the weight % of the fluorescent compound in the plastic product. The weight % of the fluorescent compound in the plastic product is then divided by the known weight % of the fluorescent compound in the marked recycled plastic which was known to have been used to make the plastic product to calculate an estimate of the recycled plastic content in the plastic product according to the following equation: The estimated value of recycled plastic content obtained from the optical Red, Green, Blue and Hue values may be combined with either or both of the estimated value of recycled plastic content from the plastic product fluorescent emission spectrum and the estimated value of recycled plastic content obtained from the plastic product fluorescent lifetime to form a further refined estimate of the recycled plastic content in the plastic product. The same can be applied to the L*, a* and b* colour space. Returning to the general manufacturing methods described herein, the marked recycled plastic is compounded with virgin plastic to produce the marked diluted plastic. Compounding the plastic components may suitably be carried out by extrusion. The process dilutes the concentration of the fluorescent compound, which was originally in the marked recycled plastic, until it is uniformly dispersed across the total marked diluted plastic. It will be appreciated that the process may be repeated until the marked diluted plastic is a homogeneous mixture of the marked recycled plastic and virgin plastic. The marked diluted plastic is then used to produce a plastic product. There is no particular limitation for the method of making the plastic product from the marked diluted plastic. It will be apparent that such production of a ‘final’ plastic product may be done at a different location, by a different party, and at a time separated significantly from the time of making the marked diluted plastic. When analysis of said plastic product is wanted, to find its recycled plastic content, a plastic product sample is taken from the plastic product for fluorescence analysis using a fluorometer. There is no particular requirement for the choice of the portion of the plastic product where the sample is taken from. There is no particular requirement for the plastic product sample size, shape or thickness. In general, a mixture of virgin and marked recycled plastic will be homogenous and so a sample can be assumed to be representative of the whole product. Preferably, the plastic product sample is of a suitable dimension to be accommodated in a sample holder of the fluorometer. The plastic product sample is placed within a fluorometer. Electromagnetic radiation of known wavelength is radiated on the plastic product sample. The emission spectrum of the plastic product sample is recorded to produce a plastic product emission spectrum. The plastic product emission spectrum sample is then compared with the standardised or reference emission spectrum, which may be a calibration curve, to determine the recycled plastic content. This process of comparison is explained in more detail above, with the discussion of product of the standardised or reference emission spectrum and calibration curve. Again, it will be appreciated that, although the plastic product sample is a small portion of the whole plastic product, the components within the plastic product (namely the fluorescent dye, the marked recycled plastic and the virgin plastic) should generally be uniformly distributed across the marked diluted plastic such that the plastic product emission spectrum is completely independent of the sampling process. Nevertheless, while not necessary for the method, it can also be possible to further extrude the sample, or melt and reform it, to ensure homogeneity before any fluorescence analysis occurs place. Again, preferably, the sample taken from the plastic product is a solid. If the sample is dissolved, the fluorescent compound may exhibit molecularly disperse emission behaviour because the diffusion time may become much shorter than the fluorescent lifetime. As discussed above, the recycled plastic content may be determined by obtaining the weight % of the fluorescent compound in the plastic product from comparison of the plastic product emission spectrum (And in particular a fluorescent intensity ratio between predetermined peaks) with the standardised or reference fluorescent emission information to obtain the recycled plastic content in the plastic product. The change in weight % of the fluorescent compound in the plastic product from the fluorescent compound in the marked recycled plastic directly correlates to bathochromic shift. The diluted concentration of the fluorescent compound in the plastic product results in a lower proportion of the aggregated (for example, dimer) form such that subsequent excitations require more energy and therefore a shorter wavelength. An increased recycled content leads to a larger bathochromic shift because a greater proportion of the fluorescent compound remains in the aggregated (for example, dimer) form. The bathochromic shift is quantified by measuring the fluorescence intensity ratio between the fluorescence intensity of the isolated fluorescent compound and the fluorescent intensity of the aggregated form in the plastic product. The standardised or reference fluorescent emission information may be of the form of a calibration curve or line. The fluorescence intensity ratio is plotted onto the calibration curve or line to obtain a weight % of the fluorescent compound in the plastic product, which directly correlates with the recycled plastic content. In another embodiment, the recycled plastic content may also be determined from comparison of the fluorescence lifetime and resulting decay function from the plastic product emission spectrum and the fluorescence lifetime and resulting decay function from the standardised or reference fluorescent emission information. The fluorescence lifetime is the measure of the amount of time the fluorescent compound spends in the excited state before emitting a photon to return to the ground state. The fluorescence lifetime also changes with aggregation of the fluorescent compound, whereby a longer fluorescence lifetime is attributed to increased aggregation, which directly correlates with an increased recycled plastic content. In another embodiment, the recycled plastic content may also be determined from colour analysis of Hue and Red, Green and Blue values of the marked recycled plastic and the plastic product under radiation of known wavelength. It will be expected that the colour of the plastic product will be blueshifted relative to the colour of the marked plastic product in the original colour image (see Figure 7; wherein vHDPE refers to virgin HDPE plastic and rHDPE to recycled HDPE plastic; marking was done using 0.25 wt% of the fluorescent compound). The colour measurements may also be performed using different colour definitions. A split over green, blue and red channels of the samples shown in Figure 7 can be seen in Figure 8. Preferably, L*a*b* colour space may be used, whereby increased aggregation may be monitored with an increase in b* values and a decrease in a* values. An example of this change can be seen in Figure 9. A greyscale comparison is shown in Figure 10. The diluted concentration of the fluorescent compound in the plastic product results in smaller proportion of the aggregated (for example, dimer) form. Subsequent excitations will then require more energy and therefore a shorter wavelength. Measurement of the change in Hue and Red, Green and Blue values from the marked recycled plastic and the plastic product can determine the recycled plastic content. It is to be appreciated that the approach described in the present specification enables the use of a variety of fluorescent compounds, including but not limited to BBS, which exhibit similar fluorescence and concentration-dependent properties to BBS, as fluorescent markers for recycled plastic: Referring to Figure 11, aggregation behaviour of 4-(2-Benzoxazolyl)-4'-(5-methyl-2-benzoxazolyl)stilbene (BMBS) is shown. Top: Fluorescence emission spectra of varying BMBS loading in HDPE, normalised to 1 at the fluorescence emission maximum of isolated BMBS molecules (~430 nm). Bottom: Resulting intensity ratios between 470, 500 and 430 nm. Fits produced using the MATLAB curve fitting toolbox. (470/430 nm R ² = 0.979 and 500/430 nm R ² = 0.974). Referring to Figure 12, aggregation behaviour of 1,4-Bis(benzo[d]oxazol-2-yl)naphthalene (BBON) is shown. Top: Fluorescence emission spectra of varying BBON loading in HDPE, normalised to 1 at the fluorescence emission maximum of isolated BBON molecules (~440 nm). Bottom: Resulting intensity ratios between 470, 500 and 430 nm. Fits produced using the MATLAB curve fitting toolbox. (470/430 nm R ² = 0.986 and 500/430 nm R ² = 0.981). Referring to Figure 13, aggregation behaviour of 1,2-Bis(5-methyl-2-benzoxazolyl)ethylene (BMBE) is shown. Top: Fluorescence emission spectra of varying BMBE loading in HDPE, normalised to 1 at the fluorescence emission maximum of isolated BMBE molecules (~420 nm). Bottom: Resulting intensity ratios between 470, 500 and 430 nm. Fits produced using the MATLAB curve fitting toolbox. (470/430 nm R ² = 0.970 and 500/430 nm R ² = 0.969). Experimental and theoretical studies of particular implementations Measurement of Recycled Content In some experiments, high-density polyethylene (HDPE) was selected as the host matrix due to its high tolerance to mechanical recycling and widespread global usage and BBS was dispersed directly in HDPE through melt extrusion at a range of concentrations (0.025-1.675 wt%) to determine minimum concentrations required for aggregation. The HDPE-BBS spectra were normalised to the fluorescence emission maxima of the isolated BBS molecules at 430 nm, corresponding to the 0 → 1 electronic transition. In doing this, any observable changes in the emission spectrum are attributed solely to dimer formation. Consequently, a new dimer band was observed at 500 nm for BBS concentrations higher than 0.025 wt% (see Figure 18). Without wishing to be bound by theory, is believed that this is due to driving BBS to the amorphous fraction of the polymer, resulting dimer formation and subsequent fluorescence changes at lower dye loadings. A higher nucleation rate was also found to reduce the minimum concentration change required to disrupt formed aggregates, which may explain why lower concentration thresholds were detected for HDPE. As described hereinafter, recycling simulations were performed by creating a BBS-HDPE master-batch (MB) (0.1 wt% BBS relative to HDPE) and diluted with virgin polymer to produce simulated recycled contents ranging from 0-100 %. Referring to Figure 14, fluorescence emission spectra of varying recycled content for HDPE recyclate marked by BBS, normalised to 1 at the fluorescence emission maxima of isolated molecules (430 nm) (top) and resulting intensity ratios between 470, 500 and 430 nm (bottom) are shown. Error bars represent the standard error (n = 5) where each sample comes from the same batch. Fits produced using the MATLAB curve fitting toolbox. (470/430 nm R ² = 0.9696 and 500/430 nm R ² = 0.9648). It can be seen from Figure 14 that the master-batch showed strong dimer-type fluorescence emission traces with notable peaks at approximately 430, 470 and 500 nm, with some degree of aggregation observed for all diluted concentrations. Explicitly, the 0.1 wt% MB BBS aggregation bands at 470 and 500 nm increased in intensity by 1.5-fold and 3.0-fold respectively between 10 and 100 % recycled content. Comparisons of dimer content were performed by calculating fluorescence intensity ratios. Spectra were normalised as reported above to remove monomer emission influence. To quantify PCR content, two different fluorescence intensity ratios were calculated between the peaks at 470 nm and 430 nm, and between 500 nm and 430 nm: Comparison of ratios rather than intensities minimises discrepancies between sample sizes and specific fluorimeter settings such as slit widths. Fluorescence intensity ratios were linear with varying recycled content and this linearity was found at MB concentrations as low as 0.025 wt%. At this BBS concentration the MB and any diluted material is visually indistinguishable from its virgin counterpart. The scaling of fluorescence intensity with BBS concentration was found to be non-linear at much higher concentrations (>0.5 wt%). Without wishing to be bound by theory, this was attributed to higher ordered structures that are believed to show aggregation-caused quenching (ACQ) .The 0.1 wt% fluorescent master-batch displayed a highly linear correlation between recycled content and intensity ratio with an R ² value of 0.9635 for the 500 nm:430 nm, and R ² of 0.9681 for 470 nm:430 nm .The high linearity between intensity ratio and recycled content of the 0.1 wt% MB paired with relatively cost-effective low dye loading confirms that this concentration could be used in an industrial setting. Further tests reported in the present specification refer to recycling simulations using a HDPE 0.1 wt% MB unless specified otherwise (see section titled “Scaling up of 0.1 wt% Master-batch” below). Verification of recycled content by fluorescence lifetimes Fluorescence lifetime measurements such as excitation wavelength, excitation duration and effects of photobleaching were found to be independent of measurement conditions. Aggregated BBS molecules were found to exhibit longer fluorescence lifetimes due to the symmetry forbidden radiative transitions involved in their fluorescence. The nature of lifetime measurements and the properties of BBS allow a secondary, precise measurement of recycled content through lifetime comparisons. Bi-exponential decay functions were found to be most suitable for the decay pattern shown by the BBS aggregates. Multi-functional exponential decay fitting equation for fluorescence emission lifetime measurements. Where a _i represents decay amplitude, τ _i represents the lifetime parameter and time, t: Bi-functional exponential decay fit for fluorescence emission lifetime measurements. Where a ₁ and b ₁ represent decay amplitudes, τ ₁ and τ ₂ represent the short and long-lived lifetime parameters respectively and time, t: Referring to Figure 15, fluorescence lifetime measurements for diluted 0.1 wt% BBS-HDPE MB are shown. Top: Fluorescence lifetime emission traces with various recycled content (o) and fitted with a bi- exponential decay function (-), excited at 340 nm and emission measurement at 500 nm. Bottom: τ ₂ – long lived lifetime parameter from bi-functional exponential function with increasing recycled content measured at 470 and 500 nm (excitation wavelength = 340 nm). Error bars represent the standard error (n = 5) where each sample comes from the same batch. Fits produced using the MATLAB curve fitting toolbox (470 nm R ² = 0.9791, 500 nm R ² = 0.9839). The initial fluorescence spectra revealed that dimer formation caused fluorescence emission shifts at 470 nm and 500 nm. Fluorescence lifetime data was therefore collected at 470 nm at an excitation wavelength of 340 nm (see Figure 15). The resulting data was indicative of dimer-type lifetimes and all variants fitted with bi-functional exponential functions (see section titled “Fitting Equation for Fluorescence Lifetimes” below). All samples showed evidence of two distinct decay times: one short lived around 1-1.5 ns ( τ ₁ ) attributed to the presence of monomer-type BBS, and a second long-lived component of approximately 10 ns (τ ₂ ) consistent with previously reported dimer decay times. All measured samples were found to show bi-functional decay curves, thus confirming some degree of dimerisation at the lowest BBS concentration of the 10 % recycled content sample (0.01 wt% BBS) (see Figure 15). The contribution of τ ₂ increased with increasing recycled content for both 470 nm and 500 nm (see Figure 15) as the ratio of dimer to monomer increased. At 470 nm, τ ₂ decreased from ~12 ns at 100 % recycled content to ~5 ns at 10 % recycled content, and from ~16 ns at 100 % recycled content to ~8 ns at 10 % recycled content at 500 nm; a similar decrease was also observed for τ ₁ . A second quantification method was realised through a linear trend for τ ₂ until 80 % recycled content, whereby a reduction in τ ₂ signifies reduced recycled content (see Figure 15). The resulting data was highly linear for both 470 and 500 nm for the 0.1 wt% MB (10 ≤ a ≤ 80), with excellent reproducibility and R ² values of 0.9791 and 0.9839 respectively (10 ≤ a ≤ 80). Practically, measurement of fluorescence lifetimes is an additional process layer to validate recyclate content, and creating a multi-layered quantification process is essential to circumvent possible falsification of recyclate incorporation through doping with fluorophores that could enhance emission at 470 and 500 nm. Tertiary recycled content determination The bathochromic shift resulting from increased dimer content increases the intensity of fluorescence emission in the green region of the visible spectrum (~550 nm) is shown in Figure 14. Under UV excitation at 365 nm, this shift is easily detectable to the naked eye. Digital analysis techniques were used to quantify visual colour changes on the 0.1 wt% BBS-HDPE MB as a potential in-house quality control method. Images were split into their corresponding colour channels using ImageJ software. An example of this process for L*a*b*, RGB (red, green, blue) and HSV (hue, saturation, value) colour spaces is depicted in Figure 16. Referring to Figure 16, colour analysis of 0.1 wt% BBS-HDPE MB samples illuminated at 365 nm and photographed in a blacked-out room is shown. (a) L*a*b* values measured using ImageJ. Fits produced using MATLAB curve fitting toolbox (L (R ² = 0.9511), a (R ² = 0.9660), b (R ² = 0.9609)). (b) RGB values measured using ImageJ. Fits produced using MATLAB curve fitting toolbox (G (R ² = 0.9718)). (c) Hue values measured using ImageJ. Fits produced using MATLAB curve fitting toolbox (Hue (R ² = 0.9511)). Errors taken as the standard deviation produced by the ImageJ software. Linear correlations were found between recycled content and a*, b*, G and hue values for the BBS-HDPE MB (see Figure 16). This procedure was developed to increase accessibility of PCR monitoring in non-specialised facilities and to provide a third tier in the quantification procedure. Unknown mixed samples could be compared to known reference standards, like the function of a universal indicator in the use of pH paper. This can allow “real-time” quick determination of recyclate content or as a quality-control check for manufacturers. The importance of quick checks is critical due to increased costs, time, and decreased convenience of specialised fluorescence equipment. Recyclate quantification through colour change can easily be realised with an appropriate standardised light-source and a camera. The accuracy of this technique could be further improved by calibrating colour values using LEDs or light sources with confirmed wavelengths and colours. Layering of quantification processes are key to minimising fraudulent marking of PCR content and ensuring reliability between sites and waste streams. Versatility and Industrial Compatibility HDPE is only one of five polymers that dominate the plastics packaging industry. Accordingly, experiments to extend the study described herein including HDPE recycled content determination to include PP and PET. Recycling simulation tests (10-100 %) were performed using a BBS-PP 0.1 wt% MB. A linear correlation was observed between recycled content and fluorescence emission ratios for PP at this BBS concentration, with intensity ratios comparable to those of HDPE under the same conditions (see Figure 19). Increases in both the short- and long-lived fluorescence lifetimes (τ ₁ and τ ₂ respectively) were due to aggregation and subsequent bathochromic shifts of BBS in PP (see Figure 21). Contrarily to results seen for HDPE and PP, the aggregation threshold of PET was found to be significantly higher as revealed by analogous loading studies Without wishing to be bound by theory, it is believed that this difference in aggregation threshold is due to the increased polarity of the PET chains with respect to the polyolefins, or due to lower crystalline content, reducing the concentration of BBS in the amorphous phase of the polymer. It was found that by annealing the PET samples at 120 °C for 5 hours, the levels of aggregation increased considerably. This coincided with crystallinity increase from ~7 % to ~40 %, thus increasing the concentration of BBS in the amorphous phase of the samples. Consequently, a higher master-batch concentration, with respect to PP and HDPE, of 0.5 wt% was trialled for PET recycling simulations. Fluorescence emission measurements initially revealed no change in the intensity of the peaks at 470 and 500 nm with increasing recycled content. Consequently, no relationship was found between recycled content and fluorescence emission ratios at both 470/430 nm and 500/430 nm for the 0.5 wt% BBS-PET prepared samples (see Figure 20), corroborating low levels of BBS aggregation. To promote aggregation, the PET samples were annealed for 5 hours at 120 °C. Post- anneal, a linear relationship, between fluorescence intensity ratios and recycled content was recorded (see Figure 20), akin to those recorded for HDPE and PP. A similar trend between annealed and non- annealed samples was observed in the corresponding fluorescence lifetime data (see Figures 22 and 23). Expansion of the technique presented herein to PP and PET can enable recycled content determination for approximately 40 % of total EU plastic demand (19.4 % PP, 12.4 % HDPE and 7.9 % PET). Through expanding the scope of this technique to multiple polymer systems and thereby covering a large proportion of the plastics market, maximum impact on facilitating recycling and increasing the average EU recycling rate of 42 % is ensured. Coloured plastics are also ubiquitous in packaging applications. Coloured plastics, specifically those that have been coloured with black pigments, notoriously cause issues in waste sorting facilities due to absorption of light coming from spectroscopic detection techniques. To ensure that use of industrially relevant polymer colourants wouldn’t negatively affect the marking technique described, this research was extended to include representative coloured plastics. Referring to Figure 17, intensity data are shown. (a) Resulting intensity ratios for red, blue, and black diluted 0.1 wt% BBS-HDPE samples between 470, 500 and 430 nm. Error bars represent the standard error (n = 5) where each sample comes from the same batch. Fits produced using the MATLAB curve fitting toolbox (470/430 nm R ² = 0.9928, 0.9786, 0.9735 (red, blue, black) and 500/430 nm R ² = 0.9773, 0.9659, 0.9716 (red, blue, black)). (b) Coloured 0.1 wt% BBS-HDPE MB samples, top: illuminated at 365 nm and photographed in a blacked-out room, and bottom: under ambient lighting. Concentrated colours (red, blue and black) in pellet form were added to a 0.1 wt% BBS-HDPE MB at 1 wt% loading to produce fluorescence coloured MBs. These fluorescence and coloured MBs were then diluted down in recycling simulations. BBS was undetectable to the naked eye in the coloured samples, yet samples fluoresced brightly under UV illumination (see Figure 17).. Visual colour analysis under UV irradiation as described above also retained linear relationships for all three of the colours tested (see Figure 27). These results further highlight the robustness of this methodology and its potential for extremely broad scope within the packaging sector and beyond. Fluorescence Emission Scaling up of 0.1 wt% Master-batch 40 g of concentrated BBS-HDPE MB was created by melt blending BBS and HDPE at 2.5 wt% in a HAAKE Minilab II micro twin-screw compounder at 200 °C with a screw speed of 100 rpm. The dyed samples were immediately quenched in a room-temperature water-bath and pelletized (2.5 mm) using a HAAKE Process 16 Varicut Pelletizer. The MB was then diluted to 0.1 wt% by melt blending the 2.5 wt% MB with virgin HDPE in a HAAKE Polylab. The dyed samples were immediately quenched in a room- temperature water-bath and pelletized (2.5 mm) using a HAAKE Process 16 Varicut Pelletizer. The MB was then compounded with virgin polymer pellets in a HAAKE Polylab to produce roughly 80 g of samples with simulated recycled contents varying from 10-100 % (maintaining processing at 200 °C and 100 rpm). The polymer-BBS pellets were individually injection moulded into dumbbells to match ISO 527- 2-1BA using a HAAKE Minijet II micro piston injection moulder. Injection moulding was completed with a cylinder temperature of 200 °C and mould temperatures of 60 °C, injection pressure of 600 bar for 5 s and a post-injection pressure of 300 bar for 5 s. The same generic process was used to produce scaled-up PP and PET masterbatches. Processing conditions for PP were the same as those for HDPE: 200 °C with a screw speed of 100 rpm with identical injection moulding conditions, cylinder temperature of 200 °C and mould temperatures of 60 °C, injection pressure of 600 bar for 5 s and a post-injection pressure of 300 bar for 5 s. For PET an extrusion temperature of 280 °C and screw speed of 100 rpm, and injection moulding conditions of 280 °C cylinder temperature, 80 °C mould temperature, injection pressure of 600 bar for 5 s and a post-injection pressure of 300 bar for 5 s. Table 1 – Tabulated BBS concentration relative to recycled content for recycling simulations of the 0.1 wt% MB of HDPE and PP: PP 0.1 wt% Recycling Simulation Referring to Figure 18, fluorescence emission spectra of varying recycled content for PP recyclate marked by BBS, normalised to 1 at the fluorescence emission maxima of isolated molecules (left) and resulting intensity ratios between 470, 500 and 430 nm for PP recycled simulations (470/430 nm R ² = 0.9957 and 500/430 nm R ² = 0.992) are shown. PET 0.5 wt% Recycling Simulation Unannealed Referring to Figure 19, fluorescence emission spectra of varying recycled content for PET recyclate marked by BBS (0.5 wt%), normalised to 1 at the fluorescence emission maxima of isolated molecules (left) and resulting intensity ratios between 470, 500 and 430 nm for PET recycled simulations (right) is shown. Annealed Referring to Figure 20, fluorescence emission spectra of varying recycled content for annealed PET recyclate marked by BBS (0.5 wt%), normalised to 1 at the fluorescence emission maxima of isolated molecules (left) and resulting intensity ratios between 470, 500 and 430 nm for PP recycled simulations (470/430 nm R ² = 0.9834 and 500/430 nm R ² = 0.9937) (right) are shown. Linear Fits for Fluorescent Emission spectra Table 2 - R ² values for truncated linear fits to intensity ratios for 470/430 nm and 500/430 nm for MB concentrations of 0.5, 0.25, 0.2 and 0.1 wt%: Fits produced using the MATLAB curve fitting toolbox according to the specified fitting ranges. PP 0.1 wt% master-batch Referring to Figure 21, fluorescence lifetime measurements for diluted 0.1 wt% BBS-PP MB are shown. (a) Fluorescence lifetime traces for diluted 0.1 wt% PP MB, excitation at 430 nm, measurement at 500 nm. (b) τ2 – long lived lifetime parameter from bi-functional exponential function with increasing recycled content measured at 470 and 500 nm with excitation wavelength of 340 nm. Fits produced using the MATLAB curve fitting toolbox (470nm R2 = 0.8871, 500nm R2 = 0.831). (c) Fluorescence lifetime traces for diluted 0.1 wt% PP MB, excitation at 430 nm, measurement at 470 nm. (d) τ2 – short lived lifetime parameter from bi-functional exponential function with increasing recycled content measured at 470 and 500 nm with excitation wavelength of 340 nm. Fits produced using the MATLAB curve fitting toolbox (470nm R2 = 0.7446, 500nm R2 = 0.9807). At 470 nm, τ ₂ decreased from ~12 ns at 100 % recycled content to ~8 ns at 10 % recycled content compared to ~12 ns at 100 % recycled content to ~5 ns at 10 % recycled content for HDPE (see Figure 21). At 500 nm these lifetimes dropped from ~14 ns at 100 % recycled content to ~9 ns at 10 % recycled content compared to from ~16 ns at 100 % recycled content to ~8 ns at 10 % recycled content for HDPE (see Figure 15). These differences in lifetime can be recognised as effects arising from the difference in interactions between BBS and the host polymer matrix, due to differences in molecular structure between HDPE and PP. PET 0.5 wt% master-batch Unannealed Referring to Figure 22, fluorescence lifetime measurements for diluted 0.5 wt% BBS-PET MB are shown. (a) Fluorescence lifetime traces for diluted 0.5 wt% PET MB, excitation at 430 nm, measurement at 500 nm. (b) τ ₂ – long lived lifetime parameter from bi-functional exponential function with increasing recycled content measured at 470 and 500 nm with excitation wavelength of 340 nm. (c) Fluorescence lifetime traces for diluted 0.5 wt% PET MB, excitation at 430 nm, measurement at 470 nm. (d) τ ₁ – short lived lifetime parameter from bi-functional exponential function with increasing recycled content measured at 470 and 500 nm with excitation wavelength of 340 nm. Annealed Referring to Figure 23, fluorescence lifetime measurements for diluted annealed 0.5 wt% BBS-PET MB are shown. (a) Fluorescence lifetime traces for diluted annealed 0.5 wt% PET MB, excitation at 430 nm, measurement at 500 nm. (b) τ ₂ – long lived lifetime parameter from bi-functional exponential function with increasing recycled content measured at 470 and 500 nm with excitation wavelength of 340 nm. (c) Fluorescence lifetime traces for diluted annealed 0.5 wt% PET MB, excitation at 430 nm, measurement at 470 nm. (d) τ ₁ – short lived lifetime parameter from bi-functional exponential function with increasing recycled content measured at 470 and 500 nm with excitation wavelength of 340 nm. Fits produced using the MATLAB curve fitting toolbox (470 nm R ² = 0.4548, 500 nm R ² = 0.7799). Similar to fluorescence emission measurements, trends in fluorescence lifetimes for the 0.5 wt% PET BBS MB were only observed in annealed samples. At 470 nm, τ ₂ stayed constant for unannealed samples ~ 6 ns at 100 % recycled content to ~ 6 ns at 10 % recycled content compared to ~8 ns at 100 % recycled content to ~6 ns at 10 % recycled content for annealed samples (see Figure 23 For the unannealed samples, τ ₂ recorded at 500 nm remained constant at ~6 ns at both 100 % and 10 % recycled content. Upon annealing, τ ₂ decreased from ~10 ns at 100 % recycled content to ~ 7 ns at 10 % recycled content for the annealed samples (see Figure 23). Colour analysis PP Referring to Figure 24, colour analysis of 0.1 wt% BBS-PP MB samples illuminated at 365 nm and photographed in a blacked-out room is shown. (a) L*a*b* values measured using ImageJ. (L (R ² = 0.8578), a (R ² = 0.8479), b (R ² = 0.8570)). (b) RGB values measured using ImageJ. (G (R ² = 0.8143)). (c) Hue values measured using ImageJ (Hue (R ² = 0.8724)). Errors taken as the standard deviation produced by the ImageJ software. Fits produced using MATLAB curve fitting toolbox. PET Referring to Figure 25, colour analysis of annealed 0.5 wt% BBS-PET MB samples illuminated at 365 nm and photographed in a blacked-out room is shown. (a) L*a*b* values measured using ImageJ. (L (R ² = 0.8530), a (R ² = 0.8435), b (R ² = 0.8506)). (b) RGB values measured using ImageJ (G (R ² = 0.8563)). (c) Hue values measured using ImageJ. (Hue (R ² = 0.8560)). Errors taken as the standard deviation produced by the ImageJ software. Fits produced using MATLAB curve fitting toolbox. Recycling Simulations of Coloured Samples Fluorescence Emission Referring to Figure 26, fluorescence emission spectra (top) and resulting intensity ratios are shown. (a) Top: Fluorescence emission spectra of varying recycled content for red HDPE recyclate marked by BBS, normalised to 1 at the fluorescence emission maxima of isolated molecules. Bottom: Resulting intensity ratios for red diluted 0.1 wt% BBS-HDPE samples between 470, 500 and 430 nm. (470/430 nm R ² = 0.9928 and 500/430 nm R ² = 0.9773. (b) Top: Fluorescence emission spectra of varying recycled content for blue HDPE recyclate marked by BBS, normalised to 1 at the fluorescence emission maxima of isolated molecules. Bottom: Resulting intensity ratios for blue diluted 0.1 wt% HDPE BBS samples between 470, 500 and 430 nm. (470/430 nm R ² = 0.9786 and 500/430 nm R ² = 0.9659. (c) Top: Fluorescence emission spectra of varying recycled content for black HDPE recyclate marked by BBS, normalised to 1 at the fluorescence emission maxima of isolated molecules. Bottom: Resulting intensity ratios for black diluted 0.1 wt% HDPE BBS samples between 470, 500 and 430 nm. (470/430 nm R ² = 0.9735 and 500/430 nm R ² = 0.9716. Error bars represent the standard error (n = 5) where each sample comes from the same batch. Fits produced using the MATLAB curve fitting toolbox. Optical analysis of Coloured Samples Referring to Figure 27 colour analysis of 0.1 wt% BBS-HDPE MB coloured samples illuminated at 365 nm and photographed in a blacked-out room is shown. (a) L*a*b*, RGB and Hue values of red samples measured using ImageJ. (b) L*a*b*, RGB and Hue values of blue samples measured using ImageJ. (c) L*a*b*, RGB and Hue values of black samples measured using ImageJ. Errors taken as the standard deviation produced by the ImageJ software. Fits produced using MATLAB curve fitting toolbox. *** The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/- 10%. Measurements Materials and Sample Preparation HDPE was provided by and manufactured by Sabic®. Sabic® HDPE B624LS is a food-grade polymer pellet of MFI of 0.5 dg/min at 190 °C/2.16 kg. Its quoted melting point is 135 °C. rHDPE, stemming from a recycled milk bottle waste stream, was provided by Alpla.4,4'-Bis(2-benzoxazolyl) stilbene (BBS), also known as Fluorescent Brightener 393 or Rylux OB-1 was supplied by Tokyo Chemical Industry UK and used as received. PET was provided by Unilever and purchased from Hardie Polymers. Food-grade virgin Ramapet N1 polymer pellets, manufactured by Indorama Ventures, with a quoted melting point of 247 ± 2 °C and of extrusion grade ([η] = 0.80 ± 2 dL/g) were used as the virgin PET. PP was purchased from Hardie Polymers. PP Capilene® T 89 E was manufactured by Carmel Olefins Ltd with a MFI of 25 g/10 min at 230 °C/2.16 kg and a quoted vicat softening temperature of 153 °C. HDPE (Sabic B624LS) pellets were dried in a fistreem vacuum oven fitted with an Edwards vacuum pump at 120°C for 16 hours to prevent hydrolytic scission during processing. Polymer-dye blends were prepared by compounding 4g of polymer with 0.005-1.675wt% (with respect to polymer matrix) of bis(benzoxazolyl)stilbene (BBS) in a HAAKE Minilab II micro twin-screw compounder at 200°C with a screw speed of 100RPM. Samples were immediately quenched in a room-temperature water-bath and pelletized using a HAAKE Process 16 Varicut Pelletizer. PET-BBS samples were subject to a second vacuum drying step before further processing. PET-BBS pellets were injection moulded into dumbbells to match ASTM D638–14 using a HAAKE Minijet II micro piston injection moulder. Injection moulding was completed with a cylinder temperature of 200°C, mould temperature of 80°C, injection pressure of 350bar for 5s and a post-injection pressure of 350bar for 5s. Dumbbells were then manually halved. Concentrated colour pellets were supplied by Colourmaster NIP Ltd in Tomato Red, Ultra Blue and Deep Black. Recycling simulations were performed on coloured samples by melt blending the 0.1 wt% BBS- HDPE-MBs with virgin polymer and including the coloured pellets at 1 wt% loading as recommended by the master-batch manufacturer in a HAAKE Polylab 16 at 200 °C and 100 rpm. Fluorescence emission spectrum characterisation Fluorescence intensity measurements were conducted at room temperature on a Cary Eclipse Fluorescence Spectrophotometer from Agilent paired with Cary Eclipse Software. Emission spectra were obtained by exciting the halved dumbbell samples at 325nm using slit widths of 2.5mm for outgoing and incoming beams and measuring emission from 350-600nm. Resulting spectra were normalised with respect to the isolated BBS molecule peak at ~430nm. The ratio of the excimer bands (~470nm and ~500nm) to isolated band (~430nm) was then determined and used as an indication to level of aggregation and subsequent recycled content (Figure 2). Any small deviation from the wavelengths is attributed to the polymer matrix polarity or specific interactions.5 samples of each sample batch were produced, and each measured once. Errors calculated by dividing standard deviation by the square root of sample number. Fluorescence lifetime experiments were performed on an Edinburgh Instruments F900 paired with F900 software. Excitations were performed using a 340nm picosecond pulsed LED with a 500ns pulse period. Emission lifetimes were measured over 200ns, de-convoluted and the resulting decay fitted to a multi- exponential decay function using non-linear least-square fitting via the F900 software. An example of fluorescence lifetime experimental data is shown in Figure 6. Additional spectroscopic quantification was performed by annealing PET samples in a Fistreem vacuum oven fitted with an R5 Edwards vacuum pump at 120°C for 16 hours prior to fluorescence measurements. Errors calculated by dividing standard deviation by the square root of sample number. Digital Photographs and Optical Analysis Optical photographs of polymer-BBS samples were taken using an IPhone XS camera under an analytikjena 6 watt UV excitation lamp on short wavelength (365nm) in a UVP Chromato viewing cabinet. ImageJ software was used to separate the resulting photos into RGB and hue stacks and the resulting values sampled 5 times across each sample, averaged and plotted against virgin:recyclate ratio. Errors taken as the standard deviation from taking the average over the pre-determined area. We herein provide the following clauses: 1. A method of quantifying the recycled plastic content of a plastic product comprising a marked recycled plastic and unmarked virgin plastic, the marked recycled plastic comprising recycled plastic and a known weight % of a fluorescent compound, and having reference fluorescent emission information; the method comprising the steps of: (a) obtaining a fluorescent emission spectrum of the plastic product to produce a plastic product fluorescent emission spectrum; (b) comparing the plastic product fluorescent emission spectrum to the reference fluorescent emission information and using that comparison to estimate the weight % of the fluorescent compound in the plastic product; (c) using the estimated weight % of the fluorescent compound in the plastic product from step (b), along with the known weight % of the fluorescent compound in the marked recycled plastic, to calculate an estimate of the recycled plastic content in the plastic product. 2. A method according to claim 1, wherein the fluorescent compound self-associates by aggregation, and wherein the isolated fluorescent compound has a first ‘isolated’ emission spectrum and wherein aggregated fluorescent compound has a second ‘aggregated’ emission spectrum, wherein at least one characteristic peak is observed at different wavelength in the first and second spectra, such that said characteristic peak is observed to shift from a first wavelength to second wavelength on aggregation of the fluorescent compound; wherein the reference fluorescent emission information is provided as a calibration curve obtained by taking a series of spectra for marked recycled plastics containing increasing weight % of the fluorescent compound and plotting the ratio of characteristic peak intensity at the first wavelength to intensity at the second wavelength; wherein step (b) comprises determining the characteristic peak ratio in the plastic product fluorescent emission spectrum and comparing that ratio to the calibration curve to obtain a reading corresponding to a known weight % of the fluorescent compound. 3. A method according to claim 1 or claim 2, wherein the fluorescent compound is one which self- associates by aggregation arising from π–π interaction. 4. A method according to any one of claims 1-3, wherein the fluorescent compound has one of the following general formulae (a) to (h): wherein each of R ¹ to R ⁷⁰ , where present, is independently selected from hydrogen, halogen, nitro, cyano, alkoxy, and substituted or unsubstituted alkyl, wherein R ⁷¹ and R ⁷² , where present, are each independently selected from hydrogen, substituted alkyl and unsubstituted alkyl; and wherein R ⁷³ , where present, is selected from hydrogen, halogen, nitro, cyano, alkoxy, and substituted or unsubstituted alkyl; and wherein R ⁵⁰ and R ⁵¹ , if present, and/or R ⁵⁶ and R ⁵⁷ , if present, may alternatively be linked to form a 6- or 7- membered heterocyclic structure. 5. A method according to claim 4, wherein the fluorescent compound is 4,4’-bis(2- benzoxazolyl)stilbene (BBS), 1,4-bis(R-cyano-4-methoxystyryl)-2,5-dimethoxybenzene (BCMDB), 1,4- bis(R-cyano-4-methoxystyryl)benzene (BCMB), 1,4-bis(R-cyano-4-(2-ethylhexyloxystyryl))-2,5- dimethoxybenzene (BCEDB) or 2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBT). 6. A method according to any one of claims 1-5, wherein the virgin plastic and recycled plastic are selected from polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), polystyrene (PS) and polypropylene (PP). 7. A method according to any one of claims 1-6, wherein the marked recycled plastic has reference fluorescent lifetime and resulting decay information, the method comprising the further steps: (a ¹ ) obtaining a fluorescent lifetime and resulting decay function of the plastic product, to produce a plastic product fluorescent lifetime and resulting decay function; and (b ¹ ) comparing the plastic product fluorescent lifetime and resulting decay function to the reference fluorescent lifetime and resulting decay information to estimate the weight % of the fluorescent compound in the plastic product; (c ¹ ) using the weight % of the fluorescent compound in the plastic product estimated in step (b ¹ ), along with the known weight % of the fluorescent compound in the marked recycled plastic, to calculate an estimate of the recycled plastic content in the plastic product; (d ¹ ) combining the estimates made in steps (c) and (c ¹ ) to form a refined estimate of the recycled plastic content in the plastic product. 8. A method according to any one of claims 1-7, wherein the marked recycled plastic has reference optical Red, Green, Blue and Hue information obtained using Red, Green, Blue channels, the method comprising the further steps: (a ² ) obtaining optical Red, Green, Blue and Hue values of the plastic product using Red, Green, Blue channels; and (b ² ) comparing the optical Red, Green, Blue and Hue values of the plastic product to the reference optical Red, Green, Blue and Hue information to estimate the weight % of the fluorescent compound in the plastic product; (c ² ) using the weight % of the fluorescent compound in the plastic product estimated in step (b ² ), along with the known weight % of the fluorescent compound in the marked recycled plastic, to calculate an estimate of the recycled plastic content in the plastic product; (d ² ) combining the estimates made in steps (c), (c ¹ ) if present and (c ² ) to form a refined estimate of the recycled plastic content in the plastic product. 9. A method of marking a recycled plastic, comprising the steps of (a) melting the recycled plastic; and then (b) mixing a fluorescent compound as a marker with the melted recycled plastic in a known weight % to produce a marked recycled plastic. 10. A method according to claim 9, further comprising the step, after step (b), of: (c) solidifying the marked recycled plastic. 11. A method according to claim 9 or claim 10, further comprising the step, after step (b) or step (c) if present, of: (d) obtaining a fluorescent emission spectrum of the marked recycled plastic to produce reference fluorescent emission information. 12. A marked recycled plastic comprising recycled plastic and a fluorescent compound. 13. A method of manufacturing a plastic product, comprising the steps of (a) compounding an unmarked virgin plastic with a marked recycled plastic according to claim 12, to make a marked diluted plastic; and (b) producing a plastic product from the marked diluted plastic. 14. A plastic product comprising a marked recycled plastic according to claim 12 and unmarked virgin plastic. 15. Use of a fluorescent compound as a marker for recycled plastics, the fluorescent compound being as set out in claim 4.