I believe that this is a midge but I would be grateful for confirmation and identification of the particular species with information on the observed colour differences (e.g. male/female).
Many groups of these were sighted last week at Stanwick Lakes in Northamptonshire, a wetland site. Each group consisted of about 100. Some landed on a wooden wall where we noticed that only one of them was green whilst the rest appeared to be black. The body length was less than 10 mm and there did not appear to be any banding on the legs.