Thanks, heisenberg; I've got it all now.
Whereas we should appreciate the effort that goes into constructing these puzzles, they do tend to gush on the Answerbank site don't they? There were a few tedious ambiguities for me which could only be resolved by checking with crossing answers. 11a could equally well be SEAR as SERE, 48d would read better with potatoes, and touch up in 11d could just as well be pat as tat. And the preamble would have been better if it had begun "Two answers clash in one cell". When I first read it, I thought every single answer had a clash and so I almost didn't bother to attempt the puzzle.