SOLVED.
Took it to an exhaust garage and they spotted the problem instantly. The join where the headers join the exhaust has a gasket which is ruined, and is hanging from one of the bolts. You could feel the blow straight away from it, and the rattling is the gasket rattling around hanging by a thread.
He's ordering a new part for me and, when I asked how much the whole job would be to fix, he quoted:
"Ohh I don't know, I mean you're talking around... £30?"
£30! To fix the problem! Dreaming
Thanks for the help though guys, much appreciated.