Well I think my girls may be doing it themselves. I went into the hive yesterday and saw a capped queen cell. Since they didn't swarm I'm guessing it's a supercedure cell. There was just one in the middle of one of the frames in the middle. I also noticed that there wasn't any uncapped larva. Plenty of capped brood, but no uncapped... so maybe they've just been in a foul mood because they've not been queenright for a while. I think I'm going to give them a couple more weeks and see how they are then before buying a queen for them.