Cheers Euan.
I had thought about the headers but the list of things that "could" be wrong with it is getting bigger the more I look into it.
I'll need to put it into a garage and see what they come up with. I'm no mechanic so if it's not staring me in the face, I'm never going to find the fault.
It's probably something easy to fix, it's just that I can't find it.