Thanks Gavin; I wasn't aware of the glenn apiaries site - so I'll bookmark it and have a good look through. I thought I'd start easily with 'Oddball Bees' but the link didn't work

Jon, I too found an unexpected queen a week or two ago. No queencells visible for me either. She was a virgin so I caged her and put her in a queenless mini-nuc in order to get mated. The following week I found a single sealed queencell in the hive so I suspect the virgin was a supercedure queen rather than one that flew in.