Thanks for all the replies, folks. I’m at work so can’t address everything in depth but:
No, not room for a bolt head on the inside. Good thinking but already been there.
Helicoil – never heard of this before, I’ll look into it.
I guess it’s a 5mm thread, same as a bottle cage and just about everything else. I think I’m probably best to continue to use 5mm bolts for future compatibility so it sounds like I want to drill, tap a bigger thread and then use one of these helicoils (Google time).
I’m in Sheffield – does anyone know where I might be able to find these (tap and die and helicoil)? As long as there’s a solution, I don’t mind a problem – generally I end up better skilled after, and I do enjoy a good bodge!