From the beginning of September: small black fly with a distinctly humped back. The wing venation, as far as I can make it out, is consistent with that for scuttle flies (Diptera: Phoridae). Indeed, there seems to be a fairly good match with the species Diplonevra nitidula (also see here). However, only a tentative identification, I think.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-09-05.