So I got the problem fixed by the independent guy for 55 quid!
He removed the short steering column shaft with the two UJ's and confirmed that the top UJ was stiff in one axis. He was able to 90% free this up by exercising it and there is now no reaction at the steering wheel. If it deteriorates again, a new shaft is £200 + fitting. We have no idea why it appeared to fail so suddenly. Perhaps the stiffness has to pass a threshold value at which it will be felt at the wheel.
Everything electrical was confirmed OK after all.
This was a weird one so thanks for your input - always good to get more than one brain on the subject. I hope this thread helps the next person with this problem.
Geoff
I love my Audi and have had several. A6 saloon, A6 Avant. Q3, Q7 and A8. Various ages and engines but all with one thing in common: a fabulous interior, quiet and refined and, for some reason, really thick paint!