Bingo. My small metal plates were too big to fit in the box around the captive nut. There's a small loop of metal under it that stops you getting anything in there! Did just about manage to jam my smallest flat bladed electrical screwrider tip in there enough to jam it up and got the bolt out! Exhaust is now all off!
Going to do all the bits I set out to do (need to feel I'm making some progress with the actual job I set out to do!) and get it back together and deal with the sheared bolt last. Hopefully it'll be in a rideable state by then to ride it to the workshop to deal with that.
Looking at the other gasket collars they are similar. I didn't realise there is a mm or two gap at the end anyway on all of them, so the end isn't in as bad condition as I thought and the main section of it is very smooth inside. Will use some sort of assembly paste too. Any particular recommendations? Unless... I've got most of a tube of
Loctite MR 5922 flange sealant left over from a previous job. It's good to +200ºc and drys flexible. Would that be suitable?