This is the Banded Hoverfly, aka the Hornet Mimic Hoverfly, Volucella zonaria (Diptera: family Syrphidae)
This is one of the largest hoverflies occurring in the UK and is most frequent in the south. It was fairly rare in the UK until about 60 years ago, but has steadily increased in numbers since and is now quite a common garden species. Because of its size and colouration, it can be mistaken for a hornet, but it is harmless, being unable to bite or sting. Females lay their eggs in the nests of social wasps, where the larvae mainly feed as scavengers but will also feed on the host larvae and pupae.
It is lways a joy when these big and blousy flies visit your garden.