I LOCKING ASSEMBLY The present invention relates to an improvement in a locking assembly that can be used as a retainer for the main pin of a shackle or the like. BACKGROUND OF THE INVENTION In a wide variety of engineering applications, there is a need to provide for releasable attachment of one member to another. Typically, a connecting pin is used that, in certain circumstances, allows for pivotal movement between the attached members. However, because such connecting pins are used in heavy engineering applications, such as dragline bucket couplings, bucket and blade couplings for earthmoving equipment, interconnection of crane boom elements etc., it is imperative that the connecting pin is secure from disengaging. For that reason, a locking or engaging means may be attached, typically to the free end of the pin, to prevent its withdrawal from the coupling member (which may, for example, be a shackle). There is a wide variety of systems and methods to lock the pin, including using spring clips, welding the locking pin and using hammers and rattle guns to assemble and disassemble the locking systems. In some instances, where large rattle guns are used to assemble the locking assembly, it is then necessary to use oxy cutting equipment for disassembly. It goes without saying that this is not only cumbersome but also greatly increases the risk of personal injury. The present invention provides a locking system that eliminates the use of hammers and rattle guns when assembling or removing the locking system, or at least provides the public with a useful alternative. SUMMARY OF THE INVENTION In a first broad aspect, the invention resides in a lock assembly (e.g. a snap lock assembly) adapted to retain a pin, the pin including a retaining notch, said lock assembly including: a body having a first cavity for the insertion of the pin therein, and a second cavity extending transversely to the first cavity into the body and positioned to communicate with said notch; and a bolt having a removable sleeve and a lip; wherein, in a first orientation (the unlocked orientation), the bolt and sleeve have a cross sectional shape correspondingly shaped to that of the second cavity and in which orientation the lip lies within the boundary provided by the circumference of the sleeve; the bolt being rotatable within the sleeve to a second orientation (the locked orientation) wherein the lip extends beyond that boundary and engages a flange in said second cavity to thereby fix the bolt in place; and the external profile of the sleeve including an edge or edges shaped so as to cause the sleeve to resist rotation when the bolt is being rotated. In preference, the second cavity has chambers at either end thereof to ensure that the bolt is wholly retained within that second cavity. In preference, the bolt includes a first flange that abuts against an indentation in the first cavity to ensure that the bolt remains within the second cavity. In preference, the bolt includes a nut to enable the bolt to be rotated within the sleeve by the use of a suitable tool. In a further broad aspect, the invention resides in a lock assembly (e.g. a snap lock assembly) adapted to fit an existing pin and cavity assembly, said lock assembly including: a bolt having a sleeve and a lip; wherein, in a first orientation (the unlocked orientation), the bolt and sleeve have a cross sectional shape correspondingly shaped to that of the cavity of the pin and cavity assembly and in which orientation the lip lies within the boundary provided by the circumference of the sleeve; the bolt being rotatable within the sleeve to a second orientation (the locked orientation) in which the lip extends beyond that boundary and engages a flange outside said cavity to thereby fix the bolt in place; and the external profile of the sleeve including an edge or edges shaped so as to cause the sleeve to resist rotation when the bolt is being rotated. The invention is not to be limited to the pin and cavity assembly described in the first aspect above, and the locking system may be varied to fit other pin and cavity assemblies, or other variations that require a locking system. In preference, the bolt includes a head that abuts against the flange outside the cavity to ensure that the bolt remains within the cavity.
j In preference, the bolt includes a nut to enable the bolt to be rotated within the sleeve by the use of a suitable tool. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of the invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings: Figure 1 is a side view of a bolt used in a lock assembly according to the present invention; Figure la is a cross-section taken at line A-A of the bolt of Figure 1; Figure 2 is a cross-sectional view of an insert that is engaged by the bolt; Figure 2a is a cross-section taken at line B-B of the lock bolt of Figure 2; Figure 3 illustrates the combination of the lock bolt and a sleeve when in an unlocked position; Figure 4 illustrates the combination as in Figure 3, but when rotated into a locked position; Figure 5 illustrates, in a cross-sectional view, the lock bolt and the retainer being inserted into the lock assembly body; Figure 6 illustrates, in a cross-sectional view, the lock bolt and the retainer fully inserted in the lock assembly body in an unlocked position; Figure 7 illustrates, in a side view, the operation of the lock assembly by rotation of the bolt through 180 degrees; and Figure 8 illustrates, in a cross-sectional view, the lock assembly when in the locked position. DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detailed description of the invention refers to the accompanying drawings. Although the description includes exemplary embodiments, other embodiments are possible, and changes may be made to the embodiments described without departing from the spirit and scope of the invention. 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 of the parts shown in the drawings may have been modified and/or exaggerated for the purposes of clarity or illustration. Turning now to the drawings, there is illustrated in Figure 1 a bolt 10 (the lock bolt) having a central cylindrical shaft 12 that has a flange 14 located proximal to one end, and also has a nut 16 located at that end. The nut 16 can be engaged by a socket or spanner or the like to cause rotation of the shaft 12. Located at the other end of the shaft is a lip or cam 18 that extends laterally outward from the shaft 12. The lock bolt 10 is typically made from a suitable metal material or composite thereof. In this embodiment, the shaft 12 has at least one longitudinal flat section 20 (shown in Figures la and 2a), and typically there are two such opposite flat sections 20 and 22 extending longitudinally along the shaft. It is to be understood that the flat sections, whose purpose will be described shortly, do not need to be separated by 180 degrees or need to extend fully along the shaft, as other configurations may also achieve the purpose of the invention. A sleeve 24 is adapted to engage the shaft 12 and, for that reason, is generally (but not necessarily) as long as the shaft. The sleeve, typically made from a hard and durable material such as plastic, has an internal and an external shape that is designed to work in conjunction with the lock bolt and a cavity of the lock body, as described below. The internal shape of the sleeve 24 includes an arc-shaped section 26, the angular extent of which equals the angular difference between the two flat sections, the sleeve then having flat internal sections 28 and 30 the width of which corresponds to the width of the flat sections on the shaft. Extending a few degrees beyond the flat sections are curved lips 32, the curvature of which corresponds to the shape of the shaft. This configuration enables the sleeve to be snapped onto the bolt. The flat sections ensure that the sleeve will usually be fixedly attached to the shaft. However, with sufficient force, through the use of a spanner or similar device, the configuration allows the shaft to rotate within the sleeve, provided of course that the sleeve is held in a non-rotatable position. Lock body 34 (see Figure 5) includes an aperture 36 that accommodates a pin 38. The pin 38 typically includes a side notch (not shown) so dimensioned and positioned that, when the lock bolt and the sleeve are inserted into a cavity 40 at the side of the body, they pass through the notch and effectively lock the pin in place. The external profile of the sleeve is configured to correspond to the cross-sectional shape of the cavity. In this embodiment, the cavity 40 is of a D-configuration, that is, having a flat side 42 and the remaining sides 44 being of a curved shape 44. Thus, the external profile of the sleeve includes a flat side 46 that corresponds to the cavity side 42. The external profile of the rest of the sleeve is not critical provided that there is an external edge 48. However, it is not intended to limit the invention to a profile having a flat edge. Other shapes may equally well be employed provided that they cause the sleeve to remain in place whilst the bolt is being rotated, thus causing the lip or cam to lock the bolt in place. Thus, a hemispherical shape may be used, as well as other possible shapes. The lateral thickness of the straight (flattened) sections of the sleeve should be selected to be sufficient that the circumference of the combination of bolt and sleeve is at least as great, if not greater, than the circumference of the shaft (including the lip or cam 18). That is, when the sleeve and the bolt are in the unlocked position (as illustrated in Figure 3), the lip or cam 18 does not extend beyond the combined cross-section of the bolt and the sleeve. The combination of the bolt and sleeve in the unlocked orientation is then inserted into the body cavity 40, the cavity also including chambers 50 and 52 at either end, the circumference of chambers 50 and 52 being greater than the circumference of the main part of the body cavity (the part of the cavity 40 that links chambers 50 and 52, as shown in Figures 5 and 6). Chamber 52 includes internal wall 54 that is abutted by flange 14 to prevent the shaft and the sleeve from extending further into the cavity. Chamber 52 also enables the lip or cam 18 to freely rotate by ensuring that the entire lip or cam is containable within the chamber 52. In the unlocked position, the bolt and the sleeve can be freely inserted into and removed out of the cavity 40. However, when the shaft is rotated through 180 degrees, the lip or cam 18 then extends laterally beyond the boundary of the cavity and engages the internal wall 54 of chamber 52. During rotation of the shaft, the sleeve is prevented from rotating by the flat external side wall engaging a correspondingly shaped wall of the cavity. The flat co-operating flat sections on the shaft and on the internal surface of the sleeve provide some degree of resistance to the shaft rotation - that resistance is too great for the shaft to be rotated by hand, but not sufficiently great to prevent rotation by the use of a suitable tool. The amount of rotation force required is something that can be calculated using well-known engineering principles - it is not the intention of this specification to provide actual dimensions. Further variations and improvements may be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived at this stage to be the most practical and preferred embodiment, it is recognised that departures may be made therefrom, whilst still remaining within the scope and spirit of the U invention. Therefore, the invention is not to be limited to the specific details disclosed herein, but is to be accorded its full scope so as to embrace any and all equivalent devices and apparatus. For example, the cross-sectional shapes of the shaft/sleeve combination and of the body cavity are not limited to those described and illustrated in the embodiment above. Many other shapes may be suitable. Furthermore, the sleeve may extend through a lesser angular range, and only one flat section on the shaft and on the internal surface of the sleeve may be needed. The lip or cam may be of different configurations. Also, the chamber 52 may be an internal groove within the lock body. The lock bolt can be used to lock into place more then one pin, and the pin(s) may each have various notches that allow it/them to extend further out from the body. Throughout this specification, except where the context requires otherwise due to express language or necessary implication, the word "comprising" is used in the sense of "including"; i.e. the specified features may be associated with further features (whether or not specifically mentioned).