Looks like a bibionid fly to me (Diptera: Bibionidae) - it doesn't look too far removed from the so called Fever Fly, Dilophus febrilis, which often occurs in very large numbers. As a group they are generally short lived as adults, but the emergence of adults will take place across 2-4 weeks - sounds like they may be at their peek in your area though. They are harmless, though obviously a nuisance - larvae feed on roots I think so can be a pest of lawns if present in high numbers.
All of the above is correct. It should be noted this is think the only bibionid in the UK that has two generations; one in spring when most bibionids are about and one at this time of year. The pictured flies are females and as this is the only Dilophus species in the U with darkened wings thre is no doubt about the identity.