A female Eudasyphora cyanella (Diptera: Muscidae). I have just noticed that the females in this species tend to have blue abdomens. The following photo, taken a month later, shows this more clearly:
In males the abdomen tends to have the same bronzy-green colour as the thorax.
Photos taken in the field below Chazey Wood, near Caversham, UK, on 2012-04-13 and 2012-05-13, respectively.