A female Chalcosyrphus nemorum (Diptera: Syrphidae). Identified using the key of Stubbs and Falk (in British Hoverflies, 2002).
A close-up of the head:
The abdomen has pairs faintish spots on tergites 2, 3, and 4:
The hind femurs are widest about 2/3 of the way from the base:
Specimen taken in the field below Chazey Wood, near Caversham, UK, on 2014-05-25.