Well, as the light levels and temperature dropped during the evening the bees started dispersing and by the time I went to bed there were only two left on the landing window. The others all found their way out but I've no idea where they went to.
I shook out bedding and towels etc just to make sure there were no stragglers before I went to bed and slept peacefully until 4.45am when I woke to the sound of buzzing!
There were a handful of bees flying around the landing and more appearing again from the gap at the top of the stairs. I risked a quick shower before the numbers built up too much and grabbed what I needed from upstairs for the day, and now the landing looks and sounds pretty much as it did yesterday! Obviously there are still more to leave the nest...
I've been reading a bit about why they swarm and this site explains it well. I've seen no sign of a queen or clustering behaviour yet, but I'm finding it fascinating. I'm wondering just how many bees are in my walls... I've got this wonderful image in my mind of my wall cavities filled with honeycomb... I wish I could see into them. I'm not sure about its insulating properties though!
So I'll let you know how this develops... It's certainly interesting, but the novelty is wearing off a bit now. And I'd really like to be able to use my own bathroom!
Update 4.45pm: Only a handful of bees upstairs now. Perhaps they only like to be up for 12 hours... I know the feeling...