Vacuum Cleaner Field of the Invention This invention relates to a vacuum cleaner. More particularly, but not exclusively, this invention relates to a filter for a vacuum cleaner. Background of the Invention Filters are key components of vacuum cleaners. Without filters, dirt and debris particles can enter the motor, reducing the efficiency and performance of the motor over time and potentially reducing the working life of the motor. In addition, dirt entering the motor may burn, producing emissions such as gases and particles which may be undesirable to be released into the atmosphere. However, in use dirt and debris particles filtered by filters tend to accumulate on the upstream side of the filter. This may block the pores of the filter reducing the ability of air to flow through the filter, thereby reducing the efficiency and performance of the vacuum cleaner. Accordingly, it is desirable to clear dirt and debris particles from the filter periodically. The present invention seeks to address at least some of these service requirements. Summary of the Invention According to a first aspect, the invention provides a vacuum cleaner comprising: a filtration chamber with an inner wall and an outer wall, an air inlet arranged to feed air carrying dust particles into the space between the inner wall and outer wall of the filtration chamber, a dirt chute for discharging dirt and/or dust from the vacuum cleaner, and a tapered filter located between the inner wall and the outer wall of the filtration chamber. The tapered filter comprises a narrow end and a wide end, and the narrow end of the filter is located at or proximate to the air inlet and/or the dirt chute. The invention provides an efficient way to remove dust and/or dirt adhered to the filter when emptying the vacuum cleaner. The taper of the filter may allow more space between the end of the filter and the wall of the air inlet or dirt chute without increasing the overall external size of the vacuum cleaner. For example, a comparable un-tapered filter would provide less clearance between the filter end and the wall of the air inlet or dirt chute, which could obstruct air flow and/or lead to dirt clogging during emptying. The increased space between the end of the filter and the wall of the air inlet or dirt chute reduces the chance of dirt clogging or being trapped between the filter and the wall of the air inlet or dirt chute. The air inlet and the dirt chute may be the same passage or the air inlet and the dirt chute may be distinct passages. The vacuum cleaner may have a longitudinal axis and the air inlet and/or the dirt chute may extend along the longitudinal axis. The outer wall may be air impermeable and the inner wall may be air permeable, such that there is an airflow path from the air inlet, through the tapered filter, and through the inner wall. The air impermeable outer wall acts to guide air within the vacuum cleaner and through the tapered filter and the air permeable inner wall allows air to be drawn through the tapered filter to be expelled from the vacuum cleaner. The inner wall may define a cavity and the cavity may house an agitation mechanism arranged to agitate dust filtered by the tapered filter to release dust from the tapered filter during emptying of the vacuum cleaner. The agitation mechanism may be employed while emptying the vacuum cleaner. Agitation of the filter may provide efficient removal of dust and/or dirt adhered to the filter when emptying the vacuum cleaner thereby increasing the efficiency and/or performance of the vacuum cleaner. The inner wall may be cylindrical and/or the outer wall may be cylindrical. The inner wall and/or the outer wall may be tapered. The outer wall may taper in an opposite direction to the tapered filter. Tapering the outer wall in an opposite direction to the taper of the tapered filter may provide a smoother airflow and/or may provide a more even distribution of airflow across the filter. The tapered filter may be thinner toward the air inlet and/or dirt chute. The tapered filter may taper in a straight line from a thick end of the tapered filter to a thin end of the tapered filter. Alternatively, the tapered filter may comprise an outer surface and an inner surface, wherein in a first portion of the tapered filter the outer surface is parallel with the inner surface and in a second portion of the tapered filter the outer surface tapers toward the inner surface. The tapered filter may have a consistent thickness from the narrow end to the wide end. The tapered filter may comprise a narrow end ring to provide structural integrity to the narrow end of the tapered filter. The tapered filter may comprise a wide end ring to provide structural integrity to the wide end of the tapered filter. The tapered filter may comprise a pleated material. The tapered filter may be a HEPA filter. The tapered filter may be a fabric filter. The tapered filter may comprise glass fibre or PTFE. The vacuum cleaner may comprise a cyclone chamber coupled to the air inlet or dirt chute. The vacuum cleaner may comprise a motor-driven impeller arranged to draw air through the tapered filter. It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a schematic representation of a vacuum cleaner; Figure 2 shows a schematic view of a filtration chamber for a vacuum cleaner; Figure 3 shows a schematic view of a filtration chamber for a vacuum cleaner; and Figure 4 shows a schematic view of a filtration chamber for a vacuum cleaner. Detailed description Figure 1 shows a vacuum cleaner 100 according to an embodiment of the invention. The vacuum cleaner 100 in the example shown is a handheld vacuum cleaner. However, in other embodiments the vacuum cleaner may be another type of vacuum cleaner, such as an upright vacuum cleaner or a canister vacuum cleaner. The vacuum cleaner 100 shown in Figure 1 may be connectable to a floor tool (not shown) in the form of a cleaner head by an elongate rigid wand (not shown). The wand may be generally tubular, the space inside forming a suction path which extends from the cleaner head to an air inlet 102 of the vacuum cleaner 100. The vacuum cleaner 100 defines a longitudinal axis 104 which runs from a front end 106 of the vacuum cleaner 100 to a rear end 108. The longitudinal axis 104 intersects the air inlet 102 of the vacuum cleaner 100. Where a wand is attached to the vacuum cleaner 100, the wand is parallel to (and in this case collinear with) the longitudinal axis 104. The vacuum cleaner 100 further comprises a pistol grip 110 which is positioned transverse to the longitudinal axis 104. The pistol grip 110 is positioned rearward of the air inlet 102, i.e. the axial position of the pistol grip 110 is further towards the rear end 108 than the air inlet 102; that is, the air inlet 102 is positioned forward of the pistol grip 110 (in that the axial position of the air inlet 102 is further towards the front end 106 than the pistol grip 110). In this embodiment, the pistol grip 110 retains a battery 112 that powers the vacuum cleaner 100. The battery 112 may be a rechargeable battery or a non-rechargeable battery. In other embodiments, the vacuum cleaner may be powered by another power source such as an external power source (for example, mains electricity supplied through a power cord). The vacuum cleaner 100 comprises a cyclone chamber 114 that, in use, is coupled to a filtration chamber 116. The cyclone chamber 114 is arranged to remove larger particles of dust or dirt (so-called “coarse dirt”). The filtration chamber 116 is coupled to a vacuum motor 118 which draws air (and dust or dirt particles) sequentially through the cyclone chamber 114 and the filtration chamber 116 before ejecting the air from an outlet at or proximate to the rear end 108 of the vacuum cleaner 100. The vacuum motor 118 may be any suitable vacuum motor. For example, the vacuum motor 118 may comprise a motor-driven impeller in which an electrical motor turns an impeller to draw air through the vacuum cleaner 100. The filtration chamber 116 has an inner wall 116a and an outer wall 116b. In the embodiment shown in Figure 1 the inner wall 116a is parallel to the longitudinal axis 104. The outer wall 116b is tapered from the air inlet 102 towards the rear end 108. In an alternative embodiment, the outer wall 116b may also be parallel with the longitudinal axis 104. In the embodiment shown in Figure 1, the inner wall 116a and the outer wall 116b are cylindrical in cross-section. In alternative embodiments, the cross-sections of the inner wall 116a and/or the outer wall 116b may not be cylindrical and may be any other suitable shape. At a front end of the filtration chamber (i.e. toward the front end 106 of the vacuum cleaner 100) the inner wall 116a and outer wall 116b define a filtration chamber air inlet 120. The filtration chamber air inlet 120 is arranged to feed air carrying dust particles into the space between the inner wall 116a and the outer wall 116b of the filtration chamber 116. Most dust and dirt particles (coarse dirt) picked up by the vacuum cleaner 100 will be deposited in the cyclone chamber 114. However, air leaving the cyclone chamber 114 and entering the filtration chamber air inlet 120 may still contain particles of dust or dirt (often smaller, lighter particles than those deposited in the cyclone chamber 114, referred to as “fine dirt”) that, as described above, can degrade the performance and/or lifetime of the vacuum motor 118. Flow of air and/or dust particles through the filtration chamber 116 of the vacuum cleaner 100 is generally indicated by the arrows in Figure 1. Air is drawn into the filtration chamber 116 through the filtration chamber air inlet 120 and directed radially outward from the longitudinal axis 104 of the vacuum cleaner 100 from where it is guided by the outer wall 116b. The outer wall 116b is air-impermeable whereas the inner wall 116a is air-permeable. Accordingly, air drawn through the vacuum cleaner 100 is guided by the outer wall 116b toward the inner wall 116a. A tapered filter (referred to hereinafter as the filter 122) is located between the inner wall 116a and the outer wall 116b of the filtration chamber 116. The inner wall 116a defines a cavity 124 that, as described below may house components for clearing dust or dirt particles from the filter 122. Air guided by the outer wall 116b is drawn through the filter 122 thereby removing at least some of the remaining dust or dirt particles (particularly the fine particles) from the air stream prior to entering the cavity 124. The filter 122 thereby acts to filter finer particles of dust or dirt from the air stream prior to the air stream meeting the vacuum motor 118 and/or any components located within the cavity 124 prolonging the life and/or efficiency of the vacuum cleaner 100 in general and the vacuum motor 118 and components located in the cavity 124 in particular. Air entering the cavity 124 is then drawn toward and through the vacuum motor 118 to be ejected from the vacuum cleaner 100. The filter 122 may comprise any suitable filtration medium. In some embodiments, the filtration medium is a so-called High-efficiency particulate air (HEPA) filtration medium. The filtration medium may comprise glass fibre or PTFE. The filter 122 may comprise a porous material such as a foam material. Typically, after use of the vacuum cleaner 100 the user is required to periodically empty material from the cyclone chamber 114 (for example, when it is full). To achieve this in the embodiment shown in Figure 1, the cyclone chamber 114 is removed from the filtration chamber 116 so that the contents of the cyclone chamber 114 may be emptied into, for example, a household waste bin. While the cyclone chamber 114 is removed from the filtration chamber 116, the user may also perform an operation to clean the filter 122. Such an operation may comprise directing the filtration chamber air inlet 120 (which then acts as a dust chute for discharging dirt and/or dust from the vacuum cleaner 100) toward a waste receptacle (such as a household waste bin) to allow dust or dirt collected by the filter 122 to fall away from the filter, thereby clearing debris from the filter 122 and maintaining efficient airflow through the filter 122. In some embodiments, removal of debris from the filter may be aided by the user manually agitating the filter 122 by, for example, tapping an exterior surface of the filtration chamber 120. In other embodiments, the cavity 124 may house an agitation mechanism 126 (as shown in Figure 1) that may be employed during emptying of the vacuum cleaner 100 that helps to remove debris that may be adhered (for example, by electrostatic forces, moisture etc.) to the filter 122. Such an agitation mechanism 126 may be activated by the user (for example, by pressing a switch or button to activate a circuit to provide electrical power to the agitation mechanism 126) or may be automatically activated by the vacuum cleaner 100 (for example, in response to detecting that the cyclone chamber 114 is removed and the filtration chamber 116 is oriented in a substantially downward configuration. A problem identified by the inventors of the present invention is that debris (i.e. dust and dirt) adhered to the filter 122 can become lodged in the filtration chamber air inlet 120 (or dirt chute as used during emptying) thereby clogging the filtration chamber air inlet 120 or dirt chute preventing efficient clearing of dust or dirt particles from the filter 122. Instead or as well, this region of the machine can provide a relatively constrictive space for air to flow, introducing turbulence and pressure drops. To address this problem the inventors of the present invention have provided a taper to the filter 122 which allows more space between the filter 122 and the inner wall of the filtration chamber air inlet 120 (or dirt chute). The increased clearance between the end of the filter 122 nearest the filtration chamber air inlet 120 (or dirt chute) and the inner wall of the filtration chamber air inlet 120 (or dirt chute) act to both reduce the obstruction of air flow through the filtration chamber air inlet 120 (or dirt chute) when the vacuum cleaner 100 is in use and/or reduce the possibility of clogging of the filtration chamber air inlet 120 (or dirt chute) during emptying and clearing of the filter 122. Figures 2 to 4 show various examples of the filtration chamber 24. Where features appear that are common to that shown in Figure 1, common reference numerals are used. Figure 2 shows a filtration chamber 200 comprising a filter 202 that tapers from a narrow end 206 located towards the filtration chamber air inlet 120 (or dirt chute) to a wide end 204 located at an opposite end of the filter 202. Accordingly, the filter 202 tapers in a straight line from the wide end 204 of the filter 202 to the narrow end 206 of the filter 202, such that the outer surface of the filter 202 is frustroconical. Figure 3 shows a filtration chamber 300 comprising a filter 302 that tapers to a narrow end 304 located towards the filtration chamber air inlet 120 (or dirt chute) from a mid-point 306 along the axial length of the filter 302 which ends at a wide end 307 located at an opposite end of the filter 302 to the narrow end 304. The filter 302 comprises an outer surface 302a and an inner surface 302b. In a first portion 308 of the filter 302 the outer surface 302a is parallel with the inner surface 302b and in a second portion 310 of the filter 302 the outer surface 302a tapers toward the inner surface 302b. Figure 4 shows a filtration chamber 400 comprising a filter 402 corresponding to the filter 202 described with reference to Figure 2. In an alternative embodiment, the filter may correspond with the filter 302 described with reference to Figure 3. Similar to the embodiment shown in Figure 1, in the embodiment shown in Figure 4, the outer wall 116b of the filtration chamber 400 tapers in an opposite direction to the taper formed by the filter 402. Tapering the outer wall 116b in an opposite direction to the taper of the filter 402 may provide a smoother airflow and/or may provide a more even distribution of airflow across the filter 402. In each of the embodiments described above, the filter may be provided with one or more structural supports to provide structural integrity to the filter (for example, to provide structural integrity while removed from the vacuum cleaner for rinsing or cleaning). For example, a narrow end ring may be included to provide structural integrity to the narrow end of the tapered filter and /or a wide end ring to provide structural integrity to the wide end of the tapered filter. Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. For example, although the tapers described above with reference to Figures 1 to 4 are linear in their change in diameter, in some embodiments, the change in diameter may be non-linear. For example, the taper may be curved. Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.