Thank you all, and BK lick who has found what is probably the original conundrum. I thought Nigel Parson got it first, and wonder if he'd ever heard it before - if not well done Nigel! This was set as a question by a Rotarian colleague for a competition, the proceeds of which are to go to charity - I will not be claiming any prize but will give my donation on behalf of Mudcat. The puzzle was driving me spare - thanks again.