Just trawling through my old threads this afternoon. I never did anything about this one as I ended up suspecting that the rear springs are too long given the car sits noticeably high at the rear, thus stretching the damper.
The bang hasn't changed in any way either circumstantially or in volume, and I've automatically adjusted my driving to deal with the odd speed hump/kerb/other thing which causes the rear left wheel to drop suddenly. Never been an MoT issue either, twice.
Dougie.