The term "gnat" is a bit vague, but it is usually used for a group of small non-biting flies, though they are closely related both to biting flies and other non-biting flies such as craneflies. Small things that bite are usually called midges or black fly depending upon their form (though in the USA black fly are sometimes called buffalo gnats, etc).

I don't know if you usually suffer so badly from midge bites, but given it is still fairly early in the season and the bites are bad they could be black fly http://en.wikipedia.org/wiki/Simuliidae some of which can have particularly unpleasant bites. The Blandford Fly is a particularly notorious blackfly, which has a wide distribution across Europe but in Britain is more or less confined to the basin of the (Dorset) Stour. http://en.wikipedia.org/wiki/Blandford_fly It is such a damn nuisance and public health problem that for some years now the council sprays a bacterium on its winter breeding areas, which kills the larvae, and has greatly reduced the problem.

You'll probably find, if it is some kind of black fly, that its season is quite short, probably just a couple of weeks. There is something that sometimes gives me nasty bites in the spring some years in my garden, but they don't hang around for long.

If it is midges, they will last all summer, if you are in a suitably damp area for midges and the weather (or marshy ground) remain suitably damp. This product will completely clear 0.5 acres to 1 acres of midges, depending upon the model you choose. There isn't any other product that does this. Unfortunately they cost the best part of £1000.

http://www.midgemonster.co.uk/products.asp?cat=13