After replacing the bolt in the topbox, this time around it's the pyramid clamp (apologies for the technical term) on the top plate which the bolt locks onto. As you can see from the pics while the bolt was wearing itself to death the pyramid clamp was also wearing itself out. (Guess which one's the original and which is new...)
The replacement parts came as a kit which can be used on custom made racks such as this one made by a guy in the states - something I may consider in the future, although I'd get one made here.
Anyway, so I've bolted the new pyramid clamp on and the box now locks on but it is still possible to physically pull the box off even when locked. It does take a fairly good pull so it's possible that it won't cause any problems - in the mean time I think I'll recheck the bolt in the box to make sure it isn't damaged and also the springs which hold the bolt out and onto the clamp. I've also bought new rubbers for the rack to hopefully hold the box in the correct position.
Really hoping that I can get this all sorted satisfactorily - starting to annoy me a tad...