I was trying to avoid damaging the thread at all costs RJ237, because i really didn't fancy having to re-thread a cylinder head thread.
It unscrewed out the bottom eventually.
I say eventually because i thought i might only have one shot at it, given the condition of the bottom of the bolt after squeezing it with pliers trying to turn it.
So i spent ages cutting a really deep clean slit in it with a hacksaw blade to get a good screwdriver grip.
I think the thread is ok, but i will check it carefully, and re-tap it if necessary.
Thanks guys, now i just need some new bolts, don't want that happening again.