I witnessed these three buff tailed bumblebees this morning on my back lawn.
Initially I thought they were mating - which is apparently seldom seen, with the large bee being a queen and the two smaller ones males. But as they're still there (30 minutes after this picture was taken) it could be two female worker bees attacking a queen which hasn't yet nested.
I've contacted the bumblebee conservation trust and posted on the wild about britain forum and will update this post if I get any replies.
Update: After writing this post, posting on the forum and sending emails the bees have now gone, maybe they've finished or the local sparrows have eaten them.
Update #2: Just had this reply from the Outreach Officer at the Bumblebee conservation trust:
The larger bumblebee, the queen is mating with one of the males. The other little male is hanging around and hoping to get involved with queen too! This photo is a brilliant example of the mating culture of bumblebees.
There is also the possibility of the picture being used in the trust's news letter.
Although it may appear to be too early for bees to be mating, it could well be an 'early bumblebee', which are short lived colonies, producing males in April and are often seldom seen after July. So a lucky find.
For more bumblebee mating photographs please see: http://viewsoftheock.blogspot.com/2011/07/perils-of-bumblebee-mating.html
Bumblebee conservation trust: http://www.bumblebeeconservation.org/bumblebees_id.htm