Hi All update
The car has now been with Audi a fourth time for this clunk when reversing.
Two different Audi dealers have look at it only because there is a dealer closer to me than where I purchase it .
Both dealers have not been able to replicate the noise not for the want of trying. On return home on two of the occasions while reversing it happened again unbelievable.
Lucky enough on one of those occasions I recorded the noise on my phone as suggested by one of the mechanics.
Audi Hereford has it at the moment trying to compare it with the S1 they have, it has been there a week so far and are about to try some different brake parts. (not convinced they are on the right track my opinion is transmission)
Can not fault either dealer on there support and help but would be nice to resolve this