The bees have a nest under the eaves and the wasps were raiding it. The battle lasted for the best part of the afternoon. I sat out on the swing seat- in the middle of the action- quite safe- like Pierre Bezukhov at Borodino- with the fighters whizzing about my ears.
Ailz tells me wasps raid hives to take bees captive and steal honey. I've no idea if this crew achieved their objective, but they're gone now and the life of the hive has returned to normal.