This is a female Bombus vestalis cuckoo bumblebee taking nectar at an Erysimum linifolium perennial wallflower. Cuckoo bumblebees can be distinguished from regular bumblebees by the lack of pollen baskets on the rear legs (bald concavities on the tibiae). This species is a brood parasite of the buff-tailed bumblebee. The female doesn't build a nest. Instead it follows the scent of a buff-tailed host to its nest and sneaks in. It then kills some of the older buff-tailed workers, lays its eggs and eventually kills the queen or forces her out of the nest. The remaining buff-tailed workers feed and look after the vestalis off-spring. This specimen was photographed in my garden in Ipswich, Suffolk on 14 April 2018. ID confirmed by Matt Smith.