The Tree Bumblebee, although common over north-eastern Europe, is a recent entrant to the UK, first being recorded in the south of England in 2001. Sometimes called New Garden Bumblebee.
Queen, workers and males have a similar appearance, with a black body and legs, a bright ginger red back of thorax and a white "tail" at the end of the abdomen.
Typical of the Tree Bumblebee early in the year (May/June); often the bright ginger fades to tawny later -- or maybe some are born lighter. Some melanistic individuals without red also exist.
These particular bees, in my garden in May 2009, were the first records of the species in that part of NW London according to the Bee, Wasp and Ant Recording Society (BWARS).
The white tail is not easy to spot in the red flower.
They typically live in holes in trees and are aggressive enough to displace other native (bumble)bees; however since their arrival they have been well-known for taking residence in bird nest boxes and here they are using a washing machine outlet in a wall.
The white tail
Inside a Tree Bumblebee nest (a rescued, relocated colony).