I apologise for not giving the preamble and clue.
I appreciate that this is very frustrating for anyone trying to help - but the theme is quite complicated to explain, and I'd really aimed the plea for help at others already doing this puzzle.
Anyway, I promise not to do it again!
Hope someone who's done it can help, all I've seen is that it's a 'doh!' moment when you get it and a bit cheeky too - not sure that's any help at all.