This is a crazy-looking dexiine tachinid. This subfamily usually have quite elongate legs but this taxon takes it to extremes with hind legs that are actually twice as long as the rest of the body. The photo doesn’t show this very well but I decided to fold the legs up to protect them on this specimen.
UPDATE (30/1/2010): After looking through the NHM collections I have concluded that this is Cholomyia inaequipes, or a very closely related species. Here are some photos of some NHM specimens:
EDIT (Monty Wood 26/9/2010): Monty examined the specimen very carefully and came to the conclusion that it probably was Cholomyia.