Talkback: The flies have it
Reply to LouLou: Without seeing the flies I can only guess, but they sound as though they may be cluster flies, Pollenia species. These mottled grey/brown flies are larger than houseflies, but smaller than blowflies. They get their name from the fact that they cluster together to overwinter, usually in lofts or attic rooms. They are known to cluster on the leeward side of houses, gusted over the roof by the wind, then dropping down and flying back into the shelter. Thousands of them can gather together. You're doing all the right things to get rid of them. You have to stop them coming in, and this may be via the loft then down through the light fitting. Although they may annoy you, don't worry, they do not spread disease.