A SURGICAL INSTRUMENT [0001] This invention relates to a surgical instrument. More specifically, although not exclusively, this invention relates to a surgical instrument for transplanting a treatment material to an organ, such as the brain, of a patient. BACKGROUND [0002] Stem cell therapies for treating neurological conditions such as Parkinson’s disease are in development, but existing delivery devices have shortcomings which render them unsuitable for use in many clinical settings. Nasal delivery and microencapsulation are not considered to work well and existing syringe-style devices are not suitable as they often require MRI scanners to be used during surgery which is often unavailable. Existing syringe- style devices are also difficult to assemble, are designed to be reusable, and are not designed for manufacturing at scale, which in turn makes existing devices economically unviable where regulation requires such delivery devices to be single-use devices. [0003] A further issue with cell therapies is the high cost and low availability of stem cells. This problem is not unique to cell therapies, and other therapies such as gene therapies need to be injected or infused into the body. The volume of treatment material, such as stem cells, available for each procedure will often be little more than the treatment dose, and as such delivery devices need to be able to deliver a very small volume of treatment material at precise locations that are often difficult to access. In the case of Parkinson’s disease the target site is within the putamen in the brain. [0004] The present invention seeks to address at least some of these issues. BRIEF SUMMARY OF THE DISCLOSURE [0005] Viewed from a first aspect, the present invention provides a surgical instrument for transplanting treatment material to an organ of a patient. The surgical instrument comprises a plunger slidably mounted to a plunger guide, the plunger comprising a sealing element mounted at a distal end of the plunger. The plunger guide comprises a hollow wire having a fluid port at a distal end and a flared proximal end for receiving the sealing element. The sealing element is arranged to form a fluid-tight seal with an inner surface of the plunger guide, such that, in use, actuating the sealing element in a first direction draws treatment material from an external source through the fluid port into an inner cavity of the hollow wire. In use, actuating the sealing element in a second direction dispenses the treatment material from the inner cavity through the fluid port. The sealing element comprises a chamfered distal end for guiding the sealing element through the flared proximal end of the plunger guide. [0006] Advantageously, the present surgical instrument can be produced at scale, is easier to assemble due to the reduced number of parts relative to existing devices and reduces the risk of damaging the sealing element during insertion and operation of the surgical instrument. Being able to manufacture the device at scale also enables the device to comply with regulatory requirements where such devices must be provided as a single-use device. [0007] A surface of the chamfered distal end of the sealing element may form an angle of between 7.5 and 9.2 degrees relative to a longitudinal axis of the sealing element. [0008] The sealing element may comprise a groove. The groove may be proximal to the chamfered distal end. This advantageously allows the sealing element to deform as it passes through the hollow wire to provide an enhanced fluid-tight seal with the hollow wire. This reduces the risk of fluid egressing proximally past the sealing element. The groove may be a circumferential groove around the sealing element. [0009] The sealing element may comprise a polymer material. The polymer material may be polytetrafluoroethylene. [0010] The hollow wire may have an internal diameter. The sealing element may have an outer diameter. The outer diameter of the sealing element may be greater than the internal diameter of the hollow wire. By oversizing the sealing element compared to the hollow wire, this provides an improved fluid-tight seal. The outer diameter of the sealing element may be up to 50µm greater than the inner diameter of the hollow wire. In some cases the outer diameter of the sealing element may be at least 20µm greater than the inner diameter of the hollow wire. The outer diameter of the sealing element may be between 20µm and 30µm greater than the inner diameter of the hollow wire. This range of interference fit has been found to provide a particularly effective fluid-tight seal without damaging the sealing element as it is displaced within the hollow wire. The outer diameter of the sealing element may be between 0.82mm and 0.86 mm. The inner diameter of the hollow wire may be 0.8mm. [0011] The flared proximal end of the plunger guide may form an angle of between 5 and 20 degrees relative to a longitudinal axis of the hollow wire. The hollow wire may comprise any of a metal material, preferably stainless steel, a polymer material or a glass material. Metal, such as stainless steel, is particularly advantageous due to uniformity in batch manufacturing and control of physical properties such as stiffness and surface finish, allow for more fine tuning of the interaction between the sealing element and the hollow wire. A polymer coating could also be included on the inner surface of the hollow wire. Stainless steel is also a surgically acceptable material. Glass cannot be easily used in certain use- cases, such as when transplanting a therapeutic agent containing cells into the brain, due to limitations in manufacturing a hollow wire small enough to minimise trauma to the brain and maintain safe mechanical and structural integrity. Metal is also preferable to polymers, as the conformity of a polymer hollow wire is more difficult to control which could result in poor performance of the device. [0012] The plunger guide may comprise a handle having a channel formed therein for receiving the sealing element. The flared proximal end may be fixed within the channel of the handle. The channel of the plunger guide handle may have a proximal section having a first diameter and a distal section having a second diameter. The second diameter may be different to the first diameter. The second diameter may be smaller than the first diameter. By counterboring the channel to provide a proximal section that is wider than the distal section, this facilitates insertion of the sealing element into the plunger guide. The flared proximal end may be fixed within the distal section of the channel. The hollow wire of the plunger guide may have a chamfered distal end. This advantageously reduces the risk of damaging the organ as the plunger guide is inserted into the organ. [0013] The plunger may comprise a handle. The plunger guide handle may be arranged to receive the plunger handle of the plunger. When the plunger handle is fully received within the plunger guide handle, the distal end of the sealing element may be aligned with the distal end of the hollow wire. This advantageously minimises the amount of treatment material required. This is particularly important where the treatment material is in very limited supply or may be dangerous to the patient. For example, the surgical instrument may be used to deliver defined doses of stem cells to the putamen in the brain. [0014] The plunger handle and plunger guide handle may have interlocking mechanical elements for affixing the plunger handle to the plunger guide handle. This advantageously prevents accidental displacement of the plunger relative to the plunger guide. The plunger handle may be affixed to the plunger guide handle at a plurality of selectable positions, such that a pre-determined dose of the treatment material is dispensed when moving between each of the plurality of positions. This is advantageous as the user is able to dispense a pre- determined dose at a specific target site and lock the device before moving the instrument to the next target site in the organ for subsequent delivery of the next dose. The instrument may be moved using any components of the stereotactic apparatus described herein. The plurality of positions may correspond to the same dose or a different dose of the treatment material. [0015] The plunger handle may comprise a protrusion arranged to engage a slot formed in the plunger guide handle. The slot may extend from a proximal position to a distal position on the plunger guide handle. [0016] The protrusion of the plunger handle may have a base portion having a first width and an end portion having a second width. The first width may be less than the second width. This advantageously avoids the base of the protrusion clashing with the plunger guide handle slot when moving the protrusion through the slot. [0017] The slot of the plunger guide handle may have a first section extending from the proximal position to the distal position. The slot may have at least one branch extending from the first section at an angle relative to the first section. At least one branch may extend substantially perpendicularly from the first section. [0018] The slot may comprise a plurality of branches spaced along the first section. The branches may be spaced equally along the first section. The plurality of branches may extend from opposed sides of the first section. The plurality of branches may extend from the same side of the first section. One or more of the branches may have a tapered profile. [0019] The slot may have an arcuate profile. The slot may have a zig zag profile. The slot profile may comprise a series of linear sections extending at an angle relative to a longitudinal axis of the handle. The slot may have a serpentine profile. The slot may have a helical profile. The helical profile may extend around the handle. The handle may include one or more abutments extending into the first section, such that the plunger handle cannot pass directly from the proximal end of the slot to the distal end of the slot along the first section. This advantageously prevents the user from accidentally dispensing more than the pre-determined dose at a given site in the organ. [0020] The plunger may comprise a wire for securing the sealing element. The wire may have a barbed end for securing the sealing element. The plunger handle may have a blind hole for receiving the plunger wire. This advantageously avoid any sharps risks due to the plunger wire. [0021] The surgical instrument may comprise a guide cannula having a proximal end for receiving the hollow wire of the plunger guide, and a distal end for directing the hollow wire to a pre-determined site in the organ. [0022] The hollow wire may be rotatable about a longitudinal axis of the surgical instrument within the guide cannula. This advantageously avoids sedimentation of the treatment material, such as cells, in the hollow wire of plunger guide when releasably mounted to the guide cannula. This is particularly relevant where the surgical instrument is not moved during prolonged periods of time during the procedure. The guide cannula may comprise a handle for receiving the plunger guide handle. When the plunger guide handle is fully received by the guide cannula handle, the distal end of the hollow wire may extend beyond the distal end of the guide cannula. [0023] The plunger guide handle may be free of protrusions such that the plunger guide can rotate relative to the guide cannula when the plunger guide handle is received by the guide cannula handle. [0024] The plunger guide handle may comprise at least one protrusion for engaging with a corresponding slot formed in the guide cannula handle to secure the plunger guide handle to the guide cannula handle. This advantageously avoids accidental separation of the plunger guide from the guide cannula when rotation of the plunger guide when fully received by the guide cannula is undesirable or unnecessary. The plunger guide handle may engage the guide cannula handle using a bayonet fitting. The slot formed in the guide cannula handle may have a first section for receiving the protrusion and a second section at an angle relative to the first section for preventing axial displacement of the plunger guide relative to the guide cannula. The second section may be substantially perpendicular to the first section. [0025] The surgical instrument may comprise a stylet releasably mounted to the guide cannula. When inserted into the guide cannula, the stylet substantially fills the volume of the guide cannula. [0026] The stylet may comprise a handle. The stylet handle and the guide cannula handle may comprise interlocking mechanical elements for affixing the guide cannula handle to the stylet handle. This prevents accidental separation of the stylet from the guide cannula during initial insertion of the guide cannula into the organ. [0027] When the stylet is releasably mounted to the guide cannula, the distal end of the stylet may protrude beyond the distal end of the guide cannula. This advantageously reduces the risk of coring the organ when inserting the assembled stylet and guide cannula initially. The stylet may comprise a wire. The wire may have a rounded distal end, for example a hemi-spherical end. The rounded distal end may have a radius of 0.6mm. This further reduces the risk of damaging the organ during insertion of the assembled stylet and guide cannula. While the therapeutic agent is transplanted (e.g. injected, deployed or otherwise introduced) into the brain in the description herein, it would be apparent this was not essential. In some cases, the organ can be part of a central nervous system of the patient. In some cases, the therapeutic agent can be deployed to one or multiple parts of the central nervous system, such as at one or more locations in the brain and/or the spinal cord of the patient. [0028] The stylet handle may comprise a polyaryletherketone material. The plunger guide handle may comprise a polyaryletherketone material. The plunger handle may comprise a polyaryletherketone material. The guide cannula handle may comprise a polyaryletherketone material. The polyaryletherketone material may comprise a polyetheretherketone material. The use of polyaryletherketone, in particular polyaryletherketone, enables the device to be manufactured in a scalable way using traditional machining methods, for example by turning a section of material on a lathe, to produce the handles described above. [0029] Viewed from a second aspect, there is also provided a kit of parts for a surgical instrument for transplanting treatment material to an organ of a patient. The kit of parts comprises a plunger guide, and a plunger slidably mountable to the plunger guide. The plunger comprises a sealing element mounted at a distal end of the plunger. The plunger guide comprises a hollow wire having a fluid port at a distal end and a flared proximal end for receiving the sealing element. The sealing element is arranged to form a fluid-tight seal with an inner surface of the plunger guide, such that, in use, actuating the sealing element in a first direction draws treatment material from an external source through the fluid port into an inner cavity of the plunger guide. In use, actuating the sealing element in a second direction dispenses the treatment material from the inner cavity through the fluid port out. The sealing element comprises a chamfered distal end for guiding the sealing element through the flared proximal end of the plunger guide. This advantageously provides a kit which can be assembled when required by the user, for example in theatre, and loaded with the treatment material. In some cases, the treatment material may comprise any of a cell- containing material, a pharmacological material and/or a radioactive substance. [0030] Viewed from a further aspect there is also provided a method of manufacturing a kit of parts for a surgical instrument for transplanting treatment material to an organ of a patient. The method comprises providing a first handle having a channel formed therein, fixing to the handle a hollow wire having a flared proximal end for receiving an external part and a distal end having a fluid port for receiving a treatment material such that the flared proximal end is fixed within the channel, providing a second handle, fixing a second wire to the second handle, and connecting a sealing element comprising a chamfered distal end to a distal end of the second wire. The sealing element is insertable into the hollow wire via the flared proximal end to sealingly engage an inner surface of the hollow wire, such that, in use, actuating the sealing element in a first direction draws treatment material from an external source through the fluid port into an inner cavity of the hollow wire. In use, actuating the sealing element in a second direction dispenses the treatment material from the inner cavity through the fluid port. [0031] There is also disclosure of use of a surgical instrument to deliver a treatment material to an organ of a patient. There is also disclosure of a surgical method comprising the steps of: providing a surgical instrument, and delivering a treatment material to an organ of a patient using the surgical instrument. [0032] There is also disclosure of a method of using a surgical instrument comprising the steps of drawing a treatment material from an external source into an inner cavity of the surgical instrument, and dispensing a pre-determined dose of the treatment material to a pre-determined site in the organ. [0033] The method may comprise introducing a guide cannula inserted into the organ of a patient, and inserting the surgical instrument into the guide cannula. [0034] Viewed from a further aspect, there is also provided a surgical apparatus for transplanting treatment material into the organ of a patient. The surgical apparatus comprises a stereotactic frame and arc; a linear actuator mounted to the stereotactic frame, and a surgical instrument mounted to the linear actuator. The frame and arc can be set to the coordinates of the target site (accounting for two degrees of freedom) while the linear actuator controls the depth of insertion (accounting for a third degree of freedom). The surgeon is then able to manually actuate the plunger relative to the plunger guide to deliver the treatment material at the target site. [0035] The surgical apparatus may comprise a guide cannula configured to receive the surgical instrument. It may be possible to rotate the plunger guide within the guide cannula. [0036] There is also disclosure of a surgical device configured as a plunger guide for use in a surgical instrument. [0037] There is also disclosure of a surgical device configured as a plunger for use in a surgical instrument. [0038] There is also disclosure of a plunger for use in a surgical instrument. The plunger comprising: a handle, and a sealing element connected to the handle and insertable into an external part having a proximal end and an inner surface corresponding to the sealing element. When inserted into the external part, the sealing element is arranged to form a fluid-tight seal with the inner surface of the external part. The sealing element comprises a chamfered distal end for guiding the sealing element through the proximal end of the external part. [0039] There is also disclosure of a plunger guide for use in a surgical instrument. The plunger guide comprises a handle having a channel formed therein extending from a proximal end of the handle to a distal end of the handle, and a hollow wire having a flared proximal end for receiving an external part and a distal end having a fluid port for receiving a treatment material. The flared proximal end is fixed to the handle within the channel. [0040] There is also disclosure of a part of a surgical instrument, the part comprising a handle having a channel, and a wire having an end disposed in the channel. The end of the wire is adhered to the handle. The handle may comprise a polyaryletherketone material. [0041] The channel may be formed from a through-hole extending from a proximal end of the handle to a distal end of the handle. The channel may be formed from a blind hole extending from a distal end of the handle to a point between a proximal end and the distal end of the handle. The channel may have a first region for receiving the wire. The channel may have a second region for receiving the adhesive. The first region may be connected to the second region such that adhesive can flow from the second region into the first region. The first region may be formed from a first hole extending from a distal end of the handle. The second region may be formed from a second hole extending from a side of the handle and intersecting the first hole. [0042] There is also disclosure of a method of manufacturing a part for a surgical instrument, the method comprising providing a handle with a channel, introducing adhesive into the channel, inserting the wire into the channel, such that the adhesive at least partially covers the wire in the channel, and curing the adhesive such that the wire is adhered to the handle. The handle may comprise a polyaryletherketone material. [0043] The surgical instrument may be configured to transplant treatment material to the brain of a patient. BRIEF DESCRIPTION OF THE DRAWINGS [0044] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Figure 1 illustrates an exemplary disassembled guide cannula and stylet assembly; Figure 2 illustrates an exemplary disassembled plunger guide and plunger assembly; Figure 3A illustrates the guide cannula; Figures 3B to 3D illustrate aspects of the guide cannula; Figure 4A illustrates the stylet; Figures 4B to 4D illustrate aspects of the stylet; Figure 5 illustrates an assembled guide cannula and stylet assembly; Figure 6 illustrates a partially assembled guide cannula and stylet assembly; Figure 7 illustrates a cross-sectional view of a part of the assembled guide cannula and stylet assembly of Figure 5; Figure 8A illustrates the plunger; Figures 8B to 8E illustrate aspects of the plunger; Figures 9A to 9E are schematic illustrations of aspects of the sealing element; Figures 10A illustrates the plunger guide; Figures 10B to 10D illustrate aspects of the plunger guide; Figures 11A to 11D are schematic illustrations of aspects of a hollow wire; Figure 12 illustrates a process of assembling a plunger guide and plunger; Figure 13 illustrates a cross-sectional view of the handle of the assembled plunger guide and plunger assembly; Figure 14 illustrates the plunger guide and plunger assembly in a loaded and an unloaded configuration; Figure 15 illustrates a cross-sectional view of the plunger guide handle with the plunger partially inserted into the plunger guide handle; Figure 16 illustrates alternative exemplary configurations of the plunger guide handle; Figure 17 illustrates a loaded plunger guide and plunger assembly partially inserted into the guide cannula with a magnified partial view showing the plunger guide and plunger assembly fully inserted into the guide cannula ; Figure 18A illustrates an assembled plunger guide and plunger assembly fully inserted into the guide cannula; Figure 18B illustrates a cross-sectional view of the guide cannula handle in the configuration of Figure 18A; Figure 19 is a schematic illustration of an exemplary oversized sealing element being inserted into a hollow wire; Figure 20A is a schematic illustration of the plunger guide and plunger assembly inserted into treatment material; Figure 20B is a schematic illustration of a plunger guide and plunger assembly having treatment material contained within the hollow wire; Figures 21A & 21B are schematic illustrations of exemplary kits of parts for a surgical instrument; Figure 22 is a schematic illustration of an exemplary method of manufacturing a kit of parts for a surgical instrument.; Figure 23 is a schematic illustration of an alternative exemplary method of manufacturing a part of a surgical instrument; Figures 24A & 24B are images of an exemplary apparatus including a stereotactic frame, a linear actuator and a surgical instrument. DETAILED DESCRIPTION [0045] The term surgical instrument is used herein to refer to devices, tools and instruments that are used in surgery. This is to distinguish from laboratory instruments which are intended to be used in a laboratory, rather than a sterile surgical setting. [0046] Figure 1 illustrates a disassembled guide cannula 100 and stylet 200 assembly. The guide cannula 100 is best illustrated in Figures 3A to 3D, which show the guide cannula 100 having a handle 105, a hollow wire 110 connected to the handle 105 at a proximal end, and extending to a distal end 115. The distal end 115 has an opening through which other components, such as the stylet 200 and the plunger guide and plunger assembly, can pass through as will be described below. The handle 105 has an opening 130 for receiving the plunger guide 400 and the stylet 200, and the opening 130 is connected to a channel 132 which extends to the distal end of the handle 105. The hollow wire 110 is fixed to the handle 105 within the channel 132. One method of fixing the hollow wire 110 to the handle 105 is to use an adhesive, such as an ethyl-based adhesive, introduced into the channel 132. A second channel 140 which intersects the first channel 132 can be provided for introducing the adhesive. A gluing port 142 can be configured to receive the nozzle of an adhesive dispenser (not shown). However, it would be apparent this was not essential, and that in some cases, the handle 105 may only have one channel 132 for receiving the adhesive and the wire 110. The proximal end of the channel 132 has a flared opening for guiding the stylet 200 or plunger guide 400 as will be explained below. The distal end of the channel 132 has a flared opening 150 to facilitate removal of any adhesive that egresses beyond the channel 132 of the handle 105. This allows for tighter control of the insertion depth of the instrument, as any adhesive which cures on the distal surface 160 would impact the insertion depth of the instrument. The illustrated hollow wire 110 has an outer diameter of 1.60mm, but this could be up to 1.68mm in some cases. As will be explained below, the outer diameter of the hollow wire 110 is under-sized relative to the internal diameter of the opening of the linear actuator 610 which receives the hollow wire 110 by 0.1mm (approximately 6%). The hollow wire 110 has an internal diameter of 1.3mm. The internal diameter may be based on any of the outer diameter of the wire 110, the outer diameter of wire 410 or the outer diameter of the stylet wire 210. The 0.3mm (approximately 19%) smaller internal diameter compared to the outer diameter of the wire 110 has been found to provide a hollow wire 110 with sufficient stiffness, enabling adequate positional accuracy of the surgical instrument whilst still being able to function with the stylet 200 and the plunger 300 and plunger guide 400 assembly. [0047] In the illustrated example, the stylet wire 210 has an outer diameter of 1.2mm and the hollow wire 110 has an internal diameter of 1.3mm. This allows the stylet 200 to be inserted and removed from the guide cannula 100 with relatively little force. By under-sizing the outer diameter of the stylet wire 210 by 0.1mm (approximately 8%) relative to the inner diameter of the hollow wire 110, this reduces the risk of coring of the brain during insertion. In some cases under-sizing the outer diameter of the stylet wire 210 by 0.2mm (approximately 15%) may be sufficient for certain clinical procedures. In some cases, it is desired to under-size the stylet wire 210 by less than 0.1mm. If the under-sizing is less than as described herein, this could result in a negative pressure being applied to the brain when the stylet 200 is withdrawn from the guide cannula 100. As shown in Figure 3D, the hollow wire 110 has a chamfered distal end 125 to reduce the sharpness of the instrument to reduce the risk of damaging the brain during insertion. The specific chamfer angle may be chosen relative to the thickness of the hollow wire 110 to provide a hollow wire which is not too blunt that it damages the brain during insertion, but also is not too sharp such that there is a risk of bleeding in the brain during insertion. The chamfered end 125 may be made by lathing or electrochemical cutting and deburring. The distal surface 160 of the handle 105 is designed to abut the holder 620 of the linear actuator 610 of the stereotactic system, and is substantially flat to ensure tight tolerance of the overall length of the guide cannula 100, and therefore the depth the stylet 200 and plunger guide and plunger assembly are inserted into the brain. The handle 105 also includes a slot 135 for receiving a corresponding protrusion 225 formed on the stylet 200 handle 205. This slot 135 has a first section extending axially along the handle 105 and a second section extending substantially perpendicular to the longitudinal axis of the handle 105, which enables the stylet 200 and the guide cannula 100 to be connected using a bayonet fitting by pressing the stylet handle 205 into the plunger guide handle 105 and turning about the longitudinal axis of the guide cannula 100 to prevent axial displacement of the stylet 200 relative to the guide cannula 100. The second section may be sufficiently long such that the stylet 100 can be rotated by 90 degrees or less to engage the guide cannula handle 105. As shown in Figure 6, the distal end 215 of the stylet 200 can be first inserted into the hollow wire 110 of the guide cannula 100 before the handle 205 is rotated to lock the stylet 200 to the guide cannula 100. [0048] The stylet 200 is best illustrated in Figures 4A to 4D, which show the stylet 200 having a handle 205, a wire 210 connected to the handle 205 at a proximal end, and extending to a distal end 215 having a rounded tip 220. Two protrusions 225 shown on opposed sides of the handle 205 are for engaging the slot 135 formed in the guide cannula handle 105. The handle 205 includes a channel 232 for receiving the wire 210 of the stylet 200. The channel 232 has a flared proximal opening 235 and a flared distal opening 240, although it would be apparent these were not essential, and in some cases there may only be one flared opening 235, 240. The handle 205 also includes a second channel 230 intersecting the first channel 232 for introducing an adhesive in a similar manner to the guide cannula handle 105 as explained above. The flared opening 240 functions in a similar manner to opening 150 and will not be repeated here. It would also be appreciated that two protrusions 225 were not essential, and in some cases, there may be only one protrusion 225. [0049] Figure 5 illustrates an assembled guide cannula and stylet assembly, showing the distal end 215 of the stylet 200 protruding beyond the distal end 115 of the guide cannula 100. In the example illustrated in Figure 5, the stylet 200 is locked to the guide cannula 100 via the bayonet fitting (see also Figure 7). In some cases, the distal surface 245 of the handle 205 abuts the guide cannula handle 105 and determines the insertion depth of the stylet 100. In one example, the distal end 115 of the guide cannula 100 is designed to be short of the target site by 25mm, with the guide cannula tip 125 being 230.2mm from the distal surface 160 of the handle 105. It would be apparent this distance was based on the specific procedure and surgical setup the surgical instrument is designed for. In the case of depositing stem cells in the putamen, a length of 230.2mm has been found suitable when the surgical instrument is used with a particular stereotactic frame, arc and linear actuator when the guide cannula 100 passes through the Electrode Holder 620 only. The specific dimensions described herein are in relation to the dimensions of the MicroDrive linear actuator with Electrode Holder and MicroGun, used as described above. It would be apparent that were a different linear actuator used, this length would need to be adjusted accordingly. In some cases, the guide cannula 100 may pass through the MicroGun 615 only, which would advantageously allow for a shorter guide cannula 100 to be used. The handle 105 is designed to abut the holder 620 such that the hollow wire 110 passes through the holder 620 and the linear actuator 610. This allows the tip 220 of the stylet 200 to protrude from the guide cannula 100. The tip 220 can be designed to protrude from the guide cannula 100 by 0.75mm to 1.5mm. As shown, the end of the stylet wire 210 protruding from the guide cannula 100 is not sharp. This is achieved by the length of stylet wire 210 protruding from the guide cannula 100 being greater than the radius of the hemispherical end 220 as shown in Figure 5. [0050] Figure 2 illustrates a disassembled plunger guide and plunger assembly. The plunger 300 is best illustrated in Figures 8A to 8E, which show the plunger 300 having a handle 305, a wire 310 connected to the handle 305 at a proximal end, and having a sealing element 315 connected to a distal end 312 of the wire 310 (see Figure 8E). The distal end 312 is barbed to securely fix the sealing element 312 to the wire 310. The wire may be turned to form the barb. It would be apparent that more than one barb may be used to secure the sealing element 315. The distal end of the wire 310 is shown having a narrower diameter than the remaining proximal portion of the wire 310. The plunger wire 310 having an outer diameter of 0.76mm has been found to provide smooth movement within the plunger guide wire 410, whilst retaining sufficient strength to avoid buckling during use and also to avoid generating excessive friction against the inner surface of the plunger guide wire 410. The handle 305 includes a protrusion 320 for engaging a slot 435 formed in the plunger guide handle 405. The protrusion 320 has a base 325 which is narrower than the distal part of the protrusion 320 which engages the slot 435 as explained below. The plunger handle 305 includes a channel 340 for receiving the wire 310. The handle 305 also includes a second channel 330 intersecting the first channel 340 for introducing an adhesive in a similar manner to the guide cannula handle 105 as explained above. The handle 305 includes a third channel 335 which acts as a glue vent to allow air to escape the channel 340 so that no air bubbles are present in the channel 340. This improves the coverage of the wire 310 and the adhesive. The wire 310 is preferably stainless steel 304, but it would be apparent this was not essential. While multiple channels 330, 335, 340 are preferred, it would be apparent this was not essential, and that one or both of the second 330 and third 335 channels may be omitted. [0051] As illustrated in Figure 8D, the sealing element 315 has a chamfered distal end 350 to facilitate insertion into the wire of the plunger guide 400. In some cases, the sealing element 315 comprises a tetrafluoroethylene material, preferably polytetrafluoroethylene. This advantageously provides a low-friction material that can still maintain the necessary fluid-tight seal with the hollow wire 410. However, it would be apparent this was not essential and that other fluoropolymers or polymers or viscoelastic materials could be used depending on the application. In the illustrated embodiment, the sealing element 315 is oversized to create an interference fit with the plunger guide wire 410 (see also Figure 19). Oversizing the sealing element 315 by approximately 20µm-30µm has been found to provide a particularly effective seal. This range of interference fit has been found to balance the expansion forces caused by the barbed end 312 within the sealing element 315 and the compressive loads generated by the interference fit of the sealing element 315 in the wire 410. The flared proximal end of the hollow wire 410 and the chamfered distal end 350 further enable an oversized sealing element 315. The sealing element 315 also includes a groove 345 around the body of the sealing element 315 to allow the sealing element 315 to deform as it is inserted into the plunger guide wire 410 to help stop the progression of any leaks passing proximally beyond the sealing element 315. While one groove 345 is shown, this was not essential and multiple grooves may be provided, particularly where a longer sealing element 315 is provided, as this may also allow for a smaller outer diameter of the sealing element 315. The barbed end 312 can be received in a channel 355 formed in the proximal end of the sealing element 315. While a single circumferential barb formed on the end 312 is shown, it would be apparent this arrangement was not essential and multiple barbs could be provided along the wire 310 to engage the sealing element 315. As shown in Figure 9A, the length L2 of the channel 355 can be 1.2mm and have a diameter D1 of 0.4mm. As shown in Figure 9B the distal-most surface of the sealing element 315 has a diameter D3 of 0.65mm. The portion of the sealing element 315 having the groove 345 formed therein may have a diameter D2 of 0.72mm. Figure 9C shows the overall length L1 of the sealing element 315 can be 2.5mm. The distance L3 from the distal-most surface of the sealing element 315 to the proximal extent of the groove 345 is 1.2mm. The distance L4 from the distal-most surface of the sealing element 315 to the distal extent of the groove 345 is 0.7mm. Thus, the groove has an axial length of 0.5mm. The axial length L5 of the chamfered surface 350 is 0.3mm. It would be apparent that the specific depths and lengths described herein are not inherently linked to the specific diameters or lengths of other portions of the sealing element 315. Figure 9D shows an end view of the sealing element 315 looking in a proximal direction, while Figures 9E shows an end view of the sealing element 315 looking in a distal direction. In both Figures, it can be seen that the outer diameter D4 of the sealing element 315 is between 0.82mm and 0.86mm ± 0.01mm. The outer diameter D4 of the sealing element 315 is based on the inner diameter D2 of the plunger guide wire 410 and the need to provide the minimum interference fit whilst minimising friction between the sealing element 315 and the plunger guide wire 410. As described herein, the outer diameter of the sealing element 315 is between 103% and 108% the inner diameter D11 of the plunger guide wire 410. Where smaller diameter sealing elements 315 are used, it has been found that treatment material may leak undesirably within the surgical instrument. Where larger diameter sealing elements 315 are used it has been found that material may be shed or torn from the sealing element 315 during insertion. Additionally, larger sealing elements 315 have been found to move poorly (e.g. not smoothly) within the plunger guide wire 410 and also to require undesirably high forces during assembly of the surgical instrument and also during delivery of the treatment material. It should be noted that the outer diameter of the sealing element 315 can be tuned depending on the required dose volume, as larger doses of treatment material may be dispensed and/or drawn more efficiently using a larger diameter hollow wire 415 and sealing element 315, rather than increasing the axial displacement of the plunger 300. [0052] The plunger guide 400 is best illustrated in Figures 10A to 10D, which show the plunger guide 400 having a handle 405, a hollow wire 410 connected to the handle 405 at a proximal end, and extending to a distal end 415. The distal end 415 has an opening 420 through which the treatment material can pass. The handle 405 has an opening 430 for receiving the plunger 300. This opening 430 is formed by drilling from the proximal end of the handle. By selecting an appropriate drill bit having a known drill taper, the opening 430 can be made to have a corresponding taper. A drill bit having a drill taper of 118 degrees has been found to be suitable for guiding the sealing element 315 during assembly. The opening 430 is connected to a channel 452 which extends to the distal end of the handle 405. The channel 452 has a proximal section 455 and a distal section 460 which is narrower than the proximal section. The wider proximal section 455 is a counterbored hole which provides support to the sealing element 315 during insertion into the plunger guide handle 405 and prevents lateral movement of the sealing element 315 during assembly. The hollow wire 410 also has a flared proximal end 412 to facilitate insertion of the sealing element 315 into the hollow wire 410. The hollow wire 410 is fixed to the handle 405 within the channel 452 such that the flared proximal end 412 is disposed in the proximal section 455 of the channel 452. As with the guide cannula 100, the hollow wire 410 can be secured to the handle 405 using an adhesive, such as an ethyl-based adhesive, introduced into a channel 465 which intersects the channel 452 containing the hollow wire 410. The distal end of the channel 452 has a flared opening 470 to facilitate removal of any adhesive that egresses beyond the distal surface of the handle 405. This allows for tighter control of the insertion depth of the instrument, as any adhesive which cures on the distal surface would impact the insertion depth of the instrument. [0053] The hollow wire 412 is best illustrated in Figures 11A to 11D. As shown in Figure 11A, the length L11 of the hollow wire is 284.4mm. As shown in Figure 11B, the axial length L12 of the chamfered end 425 is 0.1mm. The radial thickness L13 of the chamfered end 425 is 0.03mm. The particular chamfer angle of the chamfered end 425 may be dependent on the thickness of wire 410 and/or the particular surgical procedure. For example. in the described example, the putamen is relatively avascular and this chamfer angle may be less important and so the biopsy risk is low because the distal end of the assembled plunger guide 400 and plunger 300 assembly is effectively solid. As shown in Figure 11C the diameter D13 of the flared proximal end 412 is 1.32mm. As shown in Figure 11D, the flared proximal end 425 may form an angle A11 of 15 ± 5 degrees relative to a longitudinal axis of the hollow wire 410. However, in some cases angle A11 may be less than 10 degrees. For example angle A11 can be between 5 degrees and 20 degrees. The flared proximal end 412 aids assembly of the surgical instrument. The angle A11 reduces the risk of the plunger guide handle 405 becoming detached during use and also reduces the risk of damaging the sealing element 315 during insertion. Further advantages include taking only the necessary space within the handle 405 recess to accommodate the plunger wire 310. The diameter D13 is larger than the sealing element 315 diameter D4. [0054] The inner diameter D11 of the hollow wire 410 is 0.8mm ± 0.01mm, which is smaller than the outer diameter D12 by 0.3mm (approximately 37.5%). The inner surface of the hollow wire 410 preferably has a surface roughness no greater than Ra = 0.8. This assumes there would be no negative impact of providing a smoother inner surface. The inner diameter D11 of the hollow wire 410 also determines the volume in which the treatment material is contained during the procedure, and also the axial travel of the plunger 300 and also the dimensions of the plunger 300. For example, the inner diameter D11 may be chosen to provide a specific total volume in order to be able to dispense a pre-determined number of aliquots. For example, inner diameter D11 may be chosen to provide 8 aliquots. Each aliquot may be 2.5µL ± 20%. In this case, the total volume of treatment material in the described example would be 20µL ± 20% per tract. In one example, the first aliquot is deposited at the bottom of the tract, before the device is retracted by a pre-determined distance for each subsequent aliquot deposit. The procedure can then be repeated in 5 tracts per hemisphere. [0055] The outer diameter D12 of the hollow wire 410 is 1.1mm ± 0.10mm. The outer diameter D12 of the hollow wire 410 can be based on the internal diameter of wire 410 or the internal diameter of hollow wire 110. If the outer diameter D12 is smaller, there is a risk of coring the brain tissue. Conversely if a larger outer diameter D12 is chosen, there may be an insufficient gap between the guide cannula 100 and the plunger guide 400 and plunger 300 assembly. In this case, there is a risk of applying positive pressure on the brain during insertion of the surgical instrument in the guide cannula 100 and a risk the plunger guide 400 and plunger 300 assembly could contact the inner surface of the guide cannula 100. An outer diameter D12 of 1.1mm also minimises the trauma to the putamen during the procedure, where there may be five tracts created per hemisphere. In the example descried herein, the outer diameter D12 of the plunger guide wire 410 is 0.2mm smaller than the inner diameter of the hollow wire 110, but a clearance of 0.1mm would still avoid sealing between the plunger guide wire 410 and the guide cannula wire 110. [0056] Any of the handles 105, 205, 305, 405 are preferably from a polyetheretherketone material, but it would be apparent this was not essential and that other polymers could be used. Any of the wires 110, 210, 310, 410 are preferably made of stainless steel 304, but it would be apparent this was not essential and that the wires 110, 210, 310, 410 may be made of other stainless steel or metal materials. [0057] Figure 12 illustrates a process of assembling a plunger guide 400 and plunger 300. The surgical instrument formed by attaching the plunger 300 to the plunger guide 400 is intended to be assembled immediately prior to use. The flared opening 412 facilitates insertion of the plunger 300 as explained above (see also Figure 15), and the surgeon is only required to push with approximately 5N of force to introduce the plunger 300 into the plunger guide 400. As the protrusion 420 reaches the plunger guide handle 405, the surgeon can rotate the plunger 300 relative to the plunger guide 400 to align the protrusion 320 with the opening 440 in slot 435 to connect the plunger 300 to the plunger guide 400. As the protrusion passes through the first section of the slot 435, the plunger 300 can be fully inserted into the plunger guide handle 405 as shown in the lower-most configuration of Figure 12. Figure 13 illustrates a cross-sectional view of the handles 405, 305 of the assembled plunger guide and plunger assembly. When the plunger 300 is fully inserted into the plunger handle 400, the distal end 350 of the sealing element 315 aligns with the distal end 425 of the hollow wire 420. [0058] Figure 14 illustrates the plunger guide and plunger assembly in a loaded (upper image) and an unloaded (lower image) configuration. The slot 435 for receiving the protrusion 320 has a first section 445 that extends axially along the handle 405 and guides the surgeon as they depress the plunger 300 to dispense the treatment material into the brain. The slot 435 also includes branches 450 which extend from the first section 445 at a plurality of points along the length of the first section 445 and in a substantially perpendicular direction to the longitudinal axis of the handle 405. The axial position of these branches 450 corresponds to a volume within the hollow wire 420. For example, the axial length between each branch can correspond to a dose that should be dispensed at each location in the brain. Thus, in the upper image of Figure 14, the instrument can be considered to be loaded with 8 aliquots remaining, as there are 8 further distal branches 450 on the handle 405. However, it would be apparent this was not essential and that the distance between multiple branches may correspond to a specific dose. The axial travel of the plunger 300 within the plunger guide handle 405 can be based on a number of factors, including the materials and dimensions of the different components described herein. A 40mm total axial travel has been found to be effective. The provision of spaced branches 450 provides to the surgeon a visual indicator of the volume of material dispensed. By rotating the handle 305 to insert the protrusion 320 into one of the branches 450 the surgeon is prevented from accidentally dispensing more material than was intended at a given site. As shown in Figure 14 the plunger guide handle 405 has a distal portion 407 which extends distally beyond a shoulder 409. In the illustrated example, the distal portion 407 is free of any protrusions such that a loaded plunger guide and plunger assembly can be rotated when inserted into the guide cannula 100. This prevents sedimentation of the cells when loaded within the hollow wire 410. This is particularly important where the plunger guide and plunger assembly may not be moved for a prolonged period of time while other actions are being performed during the surgical procedure. [0059] Figure 16 illustrates alternative exemplary plunger guide handles having different slot configurations 465A to 465E. These configurations each create a different path for the plunger 300 and provide different indications that a given aliquot has been delivered. These configurations reduce the risk of accidental over-delivery by reducing the complexity of the path of the plunger in depositing each aliquot, and also either removing the straight channel or introducing separation between the straight channel and the aliquot locking positions. Such configurations reduce the burden on the clinician during the procedure which will further improve the safety of the procedure. While the brain has been described, it would be apparent that the present instrument could be adapted for use with other organs. [0060] Figure 17 illustrates a loaded plunger guide 400 and plunger 300 assembly partially inserted into the guide cannula 100, with a magnified partial view showing the plunger guide and plunger assembly fully inserted into the guide cannula. Once the plunger guide and plunger assembly is loaded and ready for use, the surgeon can remove the stylet 200 from the guide cannula 100 and insert the plunger guide into the guide cannula 100. As the plunger guide 400 is inserted, the shoulder 409 of the plunger guide 400 will abut a proximal face 155 of the guide cannula handle 105 (see Figure 3B and enlarged view of Figure 16) which will limit the depth of insertion of the instrument into the brain. Once the plunger guide 400 and plunger assembly is fully inserted into the guide cannula 100, the plunger guide wire 410 protrudes beyond the hollow wire 110 by 25mm ± 0.7mm. This distance is relative to the size of the target site. In the described example, where the putamen has a depth of up to 25mm, the protrusion of the plunger guide wire 410 may match the depth of the organ. If the protrusion is smaller than necessary, there is a chance the device may not reach the bottom (taken along the direction of travel of the surgical instrument in the organ) of the organ, and thus not all of the doses would be delivered at the required spacing. If the protrusion is larger than necessary, there is a risk the guide cannula 100 does not reach the target site, or the surgical instrument passes through the bottom of the target site and the treatment material is delivered outside the target site. In the illustrated example, the distal end of the hollow wire 410 is 255.2mm from the distal surface 160 of the guide cannula handle 105. As illustrated in Figures 17 & 18A, by keeping the distal portion 407 of the handle 405 free of protrusions, the plunger guide handle 405 does not interlock with the slot 135 formed in the guide cannula handle 105 and the plunger guide 400 can therefore be rotated within the guide cannula 105. [0061] Figure 20A is a schematic illustration of the plunger guide 400 and plunger 300 assembly inserted into an external fluid source containing stem cells 10. As the plunger 300 is displaced proximally relative to the plunger guide 400, the fluid-tight seal around the sealing element 315 causes the fluid containing stem cells 10 to be drawn into the cavity within the hollow wire 410, as shown in Figure 20B. The loaded instrument can then be inserted into the guide cannula 100 and doses of the cell therapy can be dispensed as described above. [0062] Figures 21A & 21B are schematic illustrations of exemplary kits of parts 500A, 500B for a surgical instrument. In kit 500A, there is a blister pack 505 which contains an assembled surgical instrument where the plunger 300 and the plunger guide 400 are in an assembled configuration. The surgeon would open the blister pack 505 and remove the assembled surgical instrument for loading as described above. In kit 500B, there is a blister pack 505 which contains a plunger 300 and plunger guide 400 in a disassembled configuration. The surgeon would open the blister pack 505, remove the plunger 300 and plunger guide 400 and insert the plunger 300 into the plunger guide 400 to form the surgical instrument as described above. [0063] Figure 22 is a schematic illustration of an exemplary method 700 of manufacturing a kit of parts for a surgical instrument. Step 705 includes providing a plunger guide handle 405 having a channel 452 formed therein. Step 710 includes fixing a hollow wire 410 having a flared proximal end 412 for receiving the sealing element 315, and a distal end 425 having a fluid port 420 for receiving a treatment material to the plunger guide handle 405 such that the flared proximal end 412 is fixed within the channel 452. Step 715 includes providing a plunger handle 305. Step 720 includes fixing a wire 310 to the plunger handle. Step 725 includes connecting a sealing element 315 comprising a chamfered distal end 350 to a distal end 312 of the plunger wire 310. [0064] Figure 23 is a schematic illustration of an alternative exemplary method of manufacturing a part for a surgical instrument. The part is any of the plunger 300, the plunger guide 400, the stylet 200 or the guide cannula 100. The method 800 includes providing 805 a handle 105, 205, 305, 405 with a channel 132, 232, 340, 452; introducing 810 adhesive into the channel 132, 232, 340, 452; inserting 815 a wire 110, 210, 310, 410 into the channel 132, 232, 340, 452, such that the adhesive at least partially covers the wire 110, 210, 310, 410 in the channel 132, 232, 340, 452, and curing 820 the adhesive such that the wire 110, 210, 310, 410 is adhered to the handle 105, 205, 305, 405. [0065] Figures 24A & 24B are images of an exemplary apparatus 600 including a stereotactic frame 607, a stereotactic arc 605, a holder 620 for holding a surgical instrument, a guide 615 to help maintain the straightness of insertion of the surgical instrument and a linear actuator 610 configured to move the surgical instrument. A suitable linear actuator 610 can comprise a MicroDrive with MicroGun 615 and Electrode Holder 620 sold by Inomed Medizintechnik GmbH. The holder 620 has an opening for receiving the surgical instrument and has an internal diameter of 1.9mm. The guide 615 has an opening for receiving the hollow wire 110 and an internal diameter of 1.70mm ± 0.02mm for the opening has been found effective for working with the presently described surgical instrument. While a target simulator is shown in the Figures, a similar apparatus can be used in surgery with a patient secured to the stereotactic frame 607. The stereotactic frame 607 allows the surgeon to position the surgical instrument at any angle in two planes relative to the patient, while the linear actuator 610 allows the surgeon to move the surgical instrument along its longitudinal axis to correctly position the distal end of the plunger wire 420 at the desired site(s) in the brain. The steps of fixing the frame 607 to the patient and drilling a bore hole in the skull of the patient are apparent to the skilled person and have not been described herein in detail. After setting up the equipment and preparing the patient for the procedure, surgeon sets the frame 600, arc 605 and linear actuator 610 (Figure 24A) to the coordinates of the delivery target, then mounts the stylet 200 and guide cannula 100 assembly to the linear actuator 610. The linear actuator 610 is fixed to the arc 605 using a clamp 625 (see Figure 24B). When the surgeon is ready, the stylet 200 can be removed from the guide cannula 100, and the loaded instrument can be inserted into the guide cannula 100. The surgeon can then dispense one or more aliquots by depressing the plunger 300 in the manner described above at the first site, before adjusting the linear actuator to move the instrument to the next position in the brain for subsequent delivery of the next dose within a single tract. When the required doses have been dispensed, the surgeon can withdraw the assembled plunger guide 400, plunger 300 and guide cannula 100 can be withdrawn from the brain. As described above, the process can be repeated for subsequent tracts. [0066] While the surgical methods described herein are in relation to loading a surgical instrument with stem cells and introducing doses of the stem cells into pre-determined sites in the putamen of the brain, it would be apparent this was merely an exemplary surgical procedure that could benefit from the present surgical instrument and surgical methods. Other surgical methods where one or more doses of treatment material, for example biological, radiological or pharmacological material, pre-loaded in a similar surgical instrument could be used to introduce the treatment material into other organs or body parts for treatment. [0067] It would also be apparent that the steps of introducing the treatment material into the surgical instrument need not be performed during surgery. In some cases the treatment material could be pre-loaded into the instrument, for example within the blister pack, further reducing the burden on the surgical team. [0068] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. [0069] Features, integers, characteristics, or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.