When I first saw this I thought it was one of those small black bees, then I saw the looped vein on its wing and realized it was a hoverfly. Only then did I look more closely and noticed the spots on its eyes. It is a female of either Eristalinus sepulchralis or Eristalinus aeneus (Diptera: Syrphidae). For more on these species see here.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-06-17.