This is one of a sub-set of flies commonly known as snipe-flies although I have not been able to establish exactly why they should have this name.
There are twelve snipe-flies in the British fauna and this one is the black-snipe fly (Chrysopilus cristatus) because it is mainly black whereas the others tend to be quite colourful. All have a tapered abdomen.
This particular species is one of the most common of the group and is found in damp meadows and woodland and this one was by the stream at Kingcombe in exactly this sort of habitat. The larvae live in leaf litter where they predate smaller insects like springtails and thrips.