A female Melanostoma scalare (Diptera: Syrphidae, order Diptera). Notice the white eggs that are visible through the sides of its abdomen. See here and here for photos by Brian Valentine that also show this. Incidentally, the halteres seem to be distinctly green in this species.
The following photo shows the same specimen from above:
Photos taken in Whiteknights Park, Reading, UK, on 2010-05-31.