[Saff]

I might be able to at last draw a line under this issue!
swapping wheels with my mate's A6 by elimination proved the issue to be with one or both of my rear pair.
I bought a set of used wheels with (barely) legal tyres and picked the best-looking pair. I swapped one and got some improvement, swapped the other and there appeared to be no vibration. Had my tyres fitted to the 'new'wheels and all seems well.
The original wheels have been checked a number of times and no-one has ever found anything wrong yet they caused this vibration. Bizarre!