Update as promised...
Apparently the fixing inside the door card (trim) is defective so a new door card is on order from Germany. This should arrive next week so I'll post a further update then.
The dealer said the buzzy resonance from the rear is not fixable, it's just the way the car is built. A bit disappointing to say the least! I've found however that setting all the sound controls to the middle and turning the surround sound up full seems to give a reasonable result, except with very loud sub-bass. I've also found that continuous hard acceleration drowns it out nicely
