Well, I put it down to the very warm weather and the abundance of nectar and pollen... the bees have swarmed *again*, the little buzzers!
The cast (as such as secondary swarm is called) was probably on Sunday 29 April - I was away. By the time I heard about it from yet another stressed neighbour, the swarm had upped and gone, with only a few stragglers left.
I looked at the hive from the outside, and it seems busy enough, so I may yet be able to produce some honey (my target is 10kg) and recover some dignity.
Bees 2, Incompetent Beekeeper 0