There are three very, very similar Syrphus species that are common in the UK. S.vitripennis, S.ribesii and S.torvus. The picture below is most likely one of these three species, and, as it it seems that the hind thighs are yellow may well be Syrphus ribesii.
If I was a better photographer and an experienced entomologist I might be more certain… on on the other hand I would probably still have to capture them and examine them magnified greatly even to dare guess! Being neither I only have to give it my best shot 🙂