In the past few months I have seen quite a lot of flies like this but I have not been able to identify them with any confidence. I think they are probably Muscid flies, possibly Helina sp. (Diptera: Muscidae), based on comparison with photos such as this and this.
The above photo shows that there is a little spine in the centre of the leading edge of each wing.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-10-04.