A male Muscid fly, probably Helina evecta (Diptera: Muscidae). Another possibility would be Helina annosa, as pointed out by Stephen R here, but according to the NBN Gateway data here and here, that species is much less common than H. evecta in the UK.
[Note added 2015-09-26: my experience collecting and identifying Muscid flies in 2014 and 2015 suggests that Helina annosa is actually quite common. Hence it quite possible that the above fly is annosa.]
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-11-05.