This very tiny fly which I happened to find on my patio door is a Simulium. It is from a genus of black flies which are between 3 and 7mm (0.1 and 0.3in) long. Not to be confused with aphids, these are biting flies which suck blood, including human blood, and which can cause serious health problems in some countries. Also referred to as biting midges, we can sometimes get plagues of them over here in the summer. It is the female that bites, and after feeding she will lay her eggs in water where the larvae will hatch. They are usually found where there is permanent or semi-permanent running water like streams and rivers. This is mostly likely a male with the larger holoptic eyes. There are several species in Britain, so it is hard to pin down the exact one without microscopic scrutiny.
September 2017, Staffordshire, England.