I Retrofit Hand Wheel Drive Assembly for Mobile Shelving Bays FIELD OF THE INVENTION [0001] The present invention relates to mobile shelving bay assemblies, in particular a hand wheel assembly that can be easily retrofitted to existing shelving bays. BACKGROUND TO THE INVENTION [0002] Mobile shelving bays are a popular storage means, occupying minimal space whilst providing ready access to all stored items. This is achieved as individual bays can be moved together to eliminate aisle space between them when not being accessed. Movement of the bays is typically facilitated by roller assemblies on the bottom of the shelving bays which run along tracks. Motive force is provided by either: a handle; a hand wheel in conjunction with a drive wheel; or an electric motor. [0003] When purchasing mobile shelving bays cost is quite often a major consideration and in order to minimise the purchase and installation cost a customer will often opt for an installation where just a handle is used to provide the motive force. Whilst useable, a system with just a handle is quite difficult to operate and the owner will often be motivated to upgrade the system by adding a hand wheel drive assembly. Such upgrades require significant reworking of the shelving bays with consequent costs. [0004] The inventor has identified the need for a cost effective hand wheel drive assembly that can be retro-fitted to shelving bays with minimal effort. The object of this invention is to provide such a hand wheel assembly, or at least provide the public with a useful alternative. SUMMARY OF THE INVENTION [0005] In a first aspect the invention comprises a supporting member; a first sprocket driven by a hand wheel at a top end of the supporting member; a second sprocket at a bottom end of the supporting member, said second sprocket being driven by a chain from the first sprocket; and a third sprocket co-axially engaged with the second sprocket, wherein the third sprocket extends beyond the bottom end of the supporting member to facilitate engagement of the sprocket with a chain to drive the mobile shelving bay. [0006] Preferably the supporting member comprises a flanged U-shaped channel to present a uniform rear surface for engagement with the mobile shelving bay. [0007] Preferably the first sprocket is of a smaller diameter than the second sprocket to provide a mechanical advantage and the third sprocket is of a larger diameter than the second sprocket to prevent interference of the chain with a track below the assembly. [0008] It should be noted that any one of the aspects mentioned above may include any of the features of any of the other aspects mentioned above and may include any of the features of any of the embodiments described below as appropriate. [0009] BRIEF DESCRIPTION OF THE DRAWINGS [0010] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows. [0011] Figure 1 shows a mobile shelving bay assembly fitted with a retrofit hand wheel drive assembly of the present invention. [0012] Figure 2 shows a front perspective view of the hand wheel drive assembly [0013] Figure 3 is an exploded view of the hand wheel drive assembly (less the hand wheel). [0014] Figures 4A and 4B show an assembled and exploded view of the top sprocket assembly of the hand wheel drive assembly. [0015] Figures 5A and 5B show an assembled and exploded view of the bottom sprocket assembly of the hand wheel drive assembly. [0016] Figure 6 shows an exploded view of the hand wheel of the hand wheel drive assembly. DETAILED DESCRIPTION OF THE INVENTION [0017] The following detailed description of the invention refers to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts. Dimensions of certain parts shown in the drawings may have been modified and/or exaggerated for the purposes of clarity or illustration. [0018] The present invention provides a hand wheel drive assembly that can be easily and quickly retrofitted to mobile shelving bay assemblies to provide motive power for moving the shelving bays. It can be attached in a matter of minutes and only requires four holes for attaching which can be unobtrusively provisioned when the shelving buys are manufactured or readily drilled when needed. [0019] Figure 1 illustrates an example shelving bay assembly 10 with the hand wheel drive assembly 20 of the present invention fitted to an end wall 12. As is well known the shelving bay includes a roller assembly 14 which moves along track 16. The track 16 includes an embedded chain (not visible) which provides traction for a sprocket 57 of the drive assembly. [0020] The essence of the hand wheel drive assembly 20 can be appreciated with the perspective view of Figure 2 in which can be seen a supporting member 21 with a hand wheel assembly 40 at the top which turns the top sprocket assembly 30 (not visible in Figure 1) which drives a chain 25 which in turn drives the bottom sprocket assembly 50 which includes a sprocket 57 extending below the supporting member to engage the track 16. The supporting member is a U-shaped channel with side flanges, allowing the subassemblies to be readily attached and still have a flat rear surface for engagement with the side wall of a shelving bay. Holes 22 are used to secure the assembly to the side of a shelving bay. [0021] Details of how the various subassemblies of the hand wheel drive assembly are connected can be seen in the exploded view of Figure 3. The top sprocket assembly 30 passes through and is attached to the supporting member 21 by screws 26 and nuts 27. The hand wheel (not shown in Figure 3) is in turn connected to the top sprocket assembly by washers 61 and 62 and nut 63. The bottom sprocket assembly 50 is attached to the supporting member by screws 28 and nuts 29. [0022] Further details of the top sprocket assembly can be seen in the assembled view of Figure 4A and the exploded view of Figure 4B. The assembly comprises a -r base plate 31 with hand wheel axle 32 to which the top chain sprocket 35 is fitted via flanged bushes 34 and 38. Circlip 33 positions the sprocket in relation to the base plate. The sprocket includes a flat 36 for engaging the hand wheel 40 and teeth 37 for driving the chain 25. [0023] Figures 5A and 5B show the bottom sprocket assembly 50 in assembled and exploded views. The assembly includes base plate 51 with axle 52 to which bottom chain sprocket 57 and track drive sprocket 56 are fitted via flanged bush 54 and held in place by circlips 53 and 58. The sprockets are co-axially engaged to transfer motive force from the chain sprocket to the track sprocket and thus from the chain to the track. The bottom chain sprocket is a greater diameter than the top chain sprocket 35 to provide a mechanical advantage. The bottom chain sprocket is smaller than the track drive sprocket to prevent interference of the chain with the track. [0024] The hand wheel assembly 40 is shown in Figure 6 and comprises hand wheel 41 and spinner 42. The spinner comprise top 45 and bottom 44 which envelope axle 43 which is attached to the hand wheel via nut 46. The top is rounded to allow the wheel to be operated by the palm of a hand. A cap 48 hides the nut 63 and washers 61, 62 used to attach the hand wheel assembly to the top sprocket assembly 30. Shroud 47 keeps the user's fingers away from the top chain sprocket 35. [0025] The reader will now appreciate the present invention which provides a hand wheel drive assembly that can be quickly easily retrofitted to mobile shelving bays. This allows a base model shelving bay system to be purchased with an easy upgrade path to a system with a hand wheel. Alternatively the hand wheel drive assembly can be fitted during the initial installation. Advantageously to the shelving bay manufacturer, provision can be made for retrofitting the hand wheel drive assembly without incurring significant cost in the base model. As only 4 holes are required in each shelving bay and a common chain fitted to the track. [0026] Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus. Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in this field. [0027] In the present specification and claims (if any), the word "comprising" and its derivatives including "comprises" and "comprise" include each of the stated integers but does not exclude the inclusion of one or more further integers.