Yesterday Zoe photographed this fly which alighted on my back during our walk round the Reading University grounds. It is probably a red-legged robber fly (Dioctria rufipes). Note the white 'beard' which is said to protect the eyes from damage by struggling prey.
[Note added 2011-03-30: The side of the thorax looks wrong for Dioctria rufipes. Probably not a Dioctria sp. ]