The Spring Flower Bee, also called the Hairy-footed Flower Bee. The pictures show the female. It is a long-tongued bee which can drink from tubular flowers.
Both male and female wake from hibernation around March and are active until May/June. The female is large and black with distinctive yellow/ginger hairs on the hind leg.
Although called "solitary" bees, the Flower Bees will often excavate their nest holes together in groups. Usually these will be horizontally in crumbling mortar or soft masonry. The one above is unusual in being vertical in the ground (beside a block of flats in north London).
Melecta albifrons, female above, is a "cuckoo" bee, a parasite, laying its eggs in nest holes of Spring Flower Bees.