I really need to use this moment to publically thank Carnut who was selected by me at random as the sounding-board for my trials and tribulations with posting puzzles.
He has been a gentleman about the whole thing, bearing in mind that my ability with the computer verges on the imbecilic. Thanks, Norman.
(PS I have spent some time on the Pros section this morning, and I can't find any answers. Now there's a shock.)