A picture truly is worth a thousand words Dan, and those that you posted are fantastic. They are a real help, thank you.
I think if I can manage to move my Cateye slightly to the left by drilling an additional hole then there will be plenty of space for the Radbot to fit alongside. I think the central hole you used will be slightly too high for the Cateye bracket however -- in its current position the light is only slightly lower than the top of the rack; any higher and it will protrude above this. I think I will prefer drilling to bridging the holes with a piece of sheetmetal as I'd like to keep the final solution as simple as possible - two pieces of metal and a plastic bracket seems a lot just to attach a rear light! Thanks for the suggestion however, and I will certainly bear this in mind should the alternatives not work for me.
I had not considered creating an entire replacement plate to replace the Thorn bracket - a great idea as it could be customized exactly to my specifications, and be made with minimum bulk. There is a sheetmetal works next to our unit at work who we are friendly with, so I am sure they would be able to knock something up for me for next to nothing, should this be necessary.
I think I need to decide upon the additional light first, and then see how they might fit on the Thorn bracket. But which one? Decisions, decisions!
Thanks for the recommendations Dan and Pete - the Blackburn does look interesting - I suddenly had images of 1982 springing into my mind - specifically Tron and Knight Rider! The tendency for it to fall off that you mention, Pete, is a concern however. And the reflector on the Radbot would greatly improve upon the one on the SKS mudguard...
I had looked at the superflashes before, didn't realise that they would fit each others' mounts. This would offer some flexibility should I get one of these in the future.
Looks like I have some more research to do!
Danny