Just seen a video locally of a large swarm a mile up the valley, unfortunately an hour ago and someone else on scene collecting them.
I was called out to one on 1st May. Earliest I know of. It was a NHS site and they were all in a tizz as bees were flying about. I advised the site manager that swarming bees have no nest to defend and are therefore more gentle than usual and that the bees would settle within 20 minutes or so. I turned up after about 1/2 hr and they had settled in someones garden on the other side of the fence, so off NHS property.
It was the first warm day for a couple of weeks so they might have been waiting to go for a few days.