Got the clunk again, today, whilst doing a three point turn, on full left lock in reverse.Decided to go into Ford dealer, who checked it out, could find nothing loose/damaged and we couldn't reproduce the clunk.
However it's going in for the clutch judder to be fixed and they will take another look for the clunk then.
Interesting that I mentioned the clutch judder. They denied knowing anything about such a problem but when I showed them posts about there being a Ford Fix for it, suddenly it was booked in for the new 'clutch pack'.