Photographed among the irises in the marsh on the northern edge of the large lake. I'm not sure of the identification as there are quite a few families of dipteran flies that contain species that look rather similar to this.
I also don't think it is a member of family Heleomyzidae, such as Suillia variegata, because they have distinct black spines on the leading edges of their wings.
It might be a member of family Sciomyzidae, such as Tetanocera elata. The latter "frequents waterside vegetation where, like other members of the family, the larvae feed on small snails" (M. Chinery, Insects of Britain & North Europe, 3rd Edition, Collins, 1993, pages 203-204). The above fly was in some irises at the edge of a lake, and I did notice several small snails on those irises.
Photo taken in Reading University grounds, Reading, UK, on 2009-05-17.