There are hundreds of species of flies in this country and identifying them is a job for the experts but a small number can be named by outright amateurs with some certainty. As always, it is a matter of not just looking at the small picture but looking at the surrounding bigger picture. However, with so many species of flies there are countless species about which we know very little.
What do we know about this species? All I can establish is that it is called Dexiosoma caninum with no common English name. The species name, caninum, obviously has something to do with dogs but the fly itself does not seem to! My field guide to the insects of Britain by Paul Brock says that this species is between 8 and 13 millimetres in length which is about half an inch which is quite big for a fly. It has long legs in relation to its body. It favours woodland and the low vegetation below the trees with a particular fondness for bracken. It can be seen from June through until September and it is believed to be a parasite of beetle larvae. Actually, that is quite a lot of information about a fly!
My photograph shows an attractively marked fly on bracken in the woodland by East Stoke fen in June. I think I can conclude from that that this is. indeed, likely to be Dexiosoma caninum.Dexiosoma caninum: a parasitic fly