What I think is a Lauxaniid fly, possibly species Minettia fasciata (Diptera: Lauxaniidae). However, this thread at diptera.info suggests that M. fasciata is rather difficult to identify unambiguously, so it might be safer to follow Paul Beuk and just assign it to the Minettia fasciata group.
The pale abdomen of the above specimen suggests it is a female (compare with the male and female in this photo). The specimen below, which has a grey abdomen, is therefore probably a male.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-07-18 (upper) and 2010-07-07 (lower).