Syrphus vitripennis... probably

There are two Syrphus species which I find are very common in parks and gardens of North London, S. vitripennis and S. ribesii. These two species are very similar; the females are easily separated by the colour of the hind femur with ribesii females having yellow hind femora whereas in vitripennis females they are black.
Males of the species vitripennis and ribesii are best separated by looking at the tiny hairs on the wings, the microtrichia.

There is another species, Syrphus rectus which has males which are indistinguishable from vitripennis males. Little is known about this species with only a few records of its existence in Britain.