Welcome to S1 ownership!
You shouldn't notice any more noise going backwards than forwards !
Is it simply the transmission clunk common to S1's or ???.
My thoughts too. Could the noise in reverse be originating from further back in the driveline, like the diff for example? Moving backwards, the sound/vibration could 'refer' forwards.
It is not uncommon for diffs to complain a bit in reverse, especially if turning. In my experience, the racier the car, the more the diff doesn't like reverse. (And an S1 is certainly at the racier end of the scale).
If you think it could be the diff, it might be worth checking what friction modifier additive the S1 diff should have, and when this was last done at service.
