[Note added 2009-06-27: I have now tentatively identified this as Coenosia humilis (family Muscidae, order Diptera), based on comparison with photos here, here, here and here. Articles here and here (pdf) mention the use of this predatory species to control pests in greenhouses.]
A dipteran fly, I have no idea of its species, genus, or even family. It was in long grass.
Photos taken in Reading University grounds, Reading, UK, on 2009-06-10.