Got the replacement "bolt" for the topbox today along with a couple of replacement springs. You can see from the pic below that the old bolt was definitely worn!
So, after the outrageous outlay of $14.50 and 2 minutes work I now have a topbox that will lock onto the bike once again.
One thing I would definitely advise all Givi owners to do is keep an eye on any movement of the topbox on the mount. The rubbers on the mount can wear and even fall out and any vibration is just going to put wear on the bolt. Replacement rubbers are not expensive either.