Sounds like the ubiquitous gnat.

Apparently there are 300 odd species in the UK.
-http://www.gardensafari.net/english/mosquitoes.htm

This is a good bit of information about the lowly gnat ~
Large mating swarms of adults often appear about dusk and may occur for several days, especially after a prolonged wet period. Many gnats are attracted to light and may be a nuisance, landing on people or entering homes or businesses. These tiny flies do not feed. They only live long enough to mate, lay eggs, and die. Eggs are laid in masses in the water or on aquatic vegetation. The life cycle usually takes about 4 to 5 weeks. There may be several generations during the summer but these insects usually disappear with the onset of dry weather. Fortunately, problems are usually temporary and intermittent.
from USA page http://www.uky.edu/Ag/Entomology/ent...ruct/ef632.htm

k :-)