I believe this hoverfly may be Syrphus torvus, understanding that it is very difficult to tell this species apart from others, particularly S. ribesii, S. vitripennis and S. rectus, and especially from a photograph.

It was the first hoverfly that I noticed in 2011, and was on the flower of a willow in the City of London Cemetery.

One aspect of the identification is that it appears to have hairs on the eyes. My guide to "British Hoverflies", by Stubbs and Falk, actually states that it is often one of the common species at sallow catkins before other species.