Most of the ones here are males, who don't sting. The females are unlikely to sting unless squashed. They're small bees, and I assume they've been enjoying the ivy that's blooming in the hedge along the allotment site boundary.
Apparently they are fairly new to the UK, having arrived only a few years ago. They are flourishing while other bees are in decline. And happily they don't harm our native bees.
I found a couple of them struggling in a water trough. I fished them out and put them in the sun to dry. One of them was off flying almost at once and didn't seem waterlogged at all.
I'm hopeful that the allotment folk will respect them and let them have these few weeks in the sun.
Many thanks to CT for educating me in the ways of the ivy mining bees.