Well, after coming across a thread in one of the other sections by chance I gave it a go and it fixed my roof thudding that occurred when going over bumpy/uneven surfaces. Was blatantly obvious when I looked at it too.

The roof bar above your head when the roof is up has an arced piece of metal at each end that folds over the top of a cylindrical bit of metal with a bolt through. It's metal to metal and if you half open the roof you can even see the abrasion in the black paint where it rubs/knocks against the bottom of this arced section.

As per the thread I stumbled on I also got hold of some sticky backed felt pads. Cost me a quid for 16 so if after time they drop off, can easily put new ones back on.

Roof thudding gone, rather annoying one at that!