Cheers lads, really appreciate the advice.

I tried the 3/8th slipped again :-(

I think I'll dremel off the head of the bolt as advised. I can see this being the best option. It's defo over torqued not siezed in there.

I'll hit the threaded shaft with the heat and wd40 and I will have space to fit in some grips too.

I'll post a piccy so as to show how the injector is held in, and how badly the access is to a silly 10mm bolt, and also the mess it's in ha ha