Difficult difficult, lemon difficult. I'm afraid an hour's scratching has only yielded 10 solutions so I'm throwing the towel in early.
I'm afraid no common device has leapt out at me and certainly nothing that makes solving this any easier. I take my hat off to the brain-boxes who are able to do these types of puzzles.
Have a great weekend everyone, hopefully tomorrow's Everyman will be mor accessible to us mere mortals!