Looking at that photo it appears to be a window fly, window gnat, sylvicola fenestralis.
Biology: Adult females lay their eggs in wet rich areas in organic matter. Often found around sewage works. The larvae which emerge are essentially aquatic, feeding on organic matter. The large numbers of these flies which can emerge from pupation areas means that these flies can constitute a nuisance in domestic premises where they are often found, as their common name suggests, on the windows.
Control: Where a large number of flies are apparent, then an attempt should be made to locate the larvae breeding site and eliminate at the source where possible. Swarms of adults maybe effectively dealt with a high residual insecticidal spray.
I'd suggest looking and spraying around pipe work, around sinks and around bins etc.
A high residual insecticide which works very well can be found at
http://www.ppcsupplies.co.uk/AG001
Best
Fred