The following is taken from the excellent Bumblebee Conservation Trust (BCT) website:

"In the UK there are 6 species of cuckoo bumblebees. These were once like other bumblebees, but they have switched to a parasitic existence. The females kill or evict the queen and take over her workers as their own, using them to rear their own. If you live in the south of England, males of the southern cuckoo bumblebee can be among the most common bumblebees in July. Cuckoo bumblebees do not have pollen baskets on their hind legs - the area is convex and covered in short hairs."

This whole insect ID thing just gets more complicated (no wonder I’ve been into birds for so long - they have distinctive songs, they’re comparatively large, they’re colourful, and you don’t have to cut them open and study their genitalia to make sure what you’re looking at…). However, having studied the BCT site and the "Field Guide to the Bumblebees of Breat Britain and Ireland" (Mike Edwards and Martin Jenner,  Ocelli  2005), I’m pretty much sure that the following photos I took on the 18th of June are of a male Southern Cuckoo Bumblebee Bombus vestalis:

• The face is short and round, typical of cuckoo bumblebees
• The bee has just one bright yellow band on the thorax
• Just above the white tail are two half bands of yellow that don’t meet in the middle
• The wings are dark

I may be talking out of my nether regions of course, but I think I’m right. Whether that matters to anyone but me is a whole other question, but frankly I’m feeling quite pleased with myself as I’d never even heard of cuckoo bumblebees until about a fortnight ago…