This spring, I have seen quite a few chunky little grey flies like one. I think it is a Leucophora sp (Diptera: Anthomyiidae). The females of these flies follow solitary bees back to their nests where they surreptitiously lay an egg when the bee is not looking. See here for more details.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2011-04-22.