The noise I am referring to is a low rumbling rattle which appears to be coming from directly underneath the passenger footwell. It only happen under specific conditions - engine warm, going uphill, usually a rough road surface, engine under pressure as I am towing a caravan, around 3000 revs. I have checked the exhaust and it appear to be rock solid, just passed mot as well. Am I looking at a baffle which develops a resonance rattle? If so are my only options to (a) ignore it or (b) replace the exhaust part? 

I hadn't considered a misfire mainly as the vibration felt through the floorpan is so significant, also there is no indication through power loss that it is a misfire, Worth considering, thanks for the suggestion. 

Heat shield on the cat secure?  Try a large hoseclip (or series of hoseclips daisychained together) to secure it all to itself and see if it goes away.