Thank you, everyone. It's the first time I've ever heard 'motto' used to mean 'motif' - sloppy puzzle setting in my view but at least I can get on now!!
Thanks for the link, ab. I tried Wickipedia and the dictionary here on the Crossword Clue Solver and neither gave that as a meaning for motto. I've always known a recurring musical phrase or theme in a book as a leit motif but you learn something new every day!!