A female Platypezid fly, probably Polyporivora picta (Diptera: Platypezidae). Similar to the female Polyporivora ornata from a few days earlier but with a black scutellum, yellow legs and a slightly different black and white pattern over the rest of the thorax and abdomen.
This is another one that I found in the same spot a few days later. The males of this species are black (see here), as are those of P. ornata.
Photos taken in the Wilderness, Whiteknights Park, Reading University grounds, Reading, UK, on 2011-10-15 and 2011-01-18, respectively.