If it were the pads freeing from the disc after being parked to go cold, there would only be one "clunk", or at least one "clunk" per side. Can you confirm how many "clunks" are being heard and also clarify:
1. Is the clunk heard only when the front wheels are turned to full lock?
2. When you first engage reverse, do you travel any backward distance with the wheels pointing straight, i.e. no lock applied?
3. And if so, is there no clunk whilst travelling straight?
4. Is the clunk apparent whether you are reversing out of the driveway to turn right, or left or both?
5. If only with the wheel rotated to turn left, or right, which lock is being applied when the clunk appears, i.e. left hand down or right hand down?
6. If you look into the wheel arch of the nearside front wheel, use a torch, can you see any indications where the leading edge of the turned wheel may have contacted the wheel arch?
N6 JMX