What sort of chance does somebody have of getting a phd position in chemistry at a top uni if they have done their degree at an ex-poly uni. Am I wasting my time.

It's certainly worth applying. You probably stand less chance than somebody with a first from Oxford or Cambridge, but so do most people.

Good luck

I was at a former poly and I got a PhD position in one of the best biochemistry departments in the country at a very good university so I wouldn't worry too much about it.