Took the car for a longer run and the exhaust seemed to be definitely getting louder. It was only when I stopped off at work that the rattling was noticeable. Bonnet off and it was quickly spotted that there were only 2 exhaust nuts (out of 8!) holding the manifold on. They were definitely tight (FT) before but I can only guess that the vibration has got to them.
I've replaced them all (and gone to VFT) but it's something to keep an eye on in the future.