A rather grey female Eristalis arbustorum (Diptera: Syrphidae). This must be one of females in which the orange markings are absent "resulting in some females being black with only thin white apical rims on each tergite" (Stubbs and Falk, British Hoverflies, page 289).
"The presence of a completely pale-dusted face distinguishes this species very easily." (also Stubbs and Falk, page 289). See here for another face-on view of a female of this species.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2011-07-23.