Syritta pipiens
 

A handful of this  small species of hoverfly flitted around in Ray Hamblett's south Lancing garden. This is an urban species associated with compost where its larvae live.  Syritta pipiens Identification by   Matt Smith on UK Hoverflies

Identification notes:

The head is black in this species whereas Platycheirus is the usual maroon colour.
The scutellum is black whereas in Sphaerophoria it is yellow
The front leg colour does not match the Platycheirus when compared with the thorax patterns and head colour
Swollens legs seems to indicate the genus Xylota
Swollen legs also seem to be found in Chalcosyrphus nemorum
Wrong habitat for Chalcosyrphus nemorum