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The cluster flies are the genus Pollenia in the blowfly family Calliphoridae. Unlike more familiar blow flies, such as the bluebottle genus Phormia, they do not present a health hazard because they do not lay eggs in human food. They are strictly parasitic on earthworms; the females lay their eggs near earthworm burrows, and the larvae then infest the worms. However, the flies are a nuisance; when the adults emerge in the late summer or autumn, they enter houses to hibernate, often in large numbers; they are difficult to eradicate because they favour inaccessible spaces such as roof and wall cavities. They are often seen on windows of little-used rooms. They are also sometimes known as attic flies.
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Any cracks at all in brick, or around windows or in soffits, and the young flies will get in. As soon as the warm weather hits, we have to vacuum them up daily.
Black-flies are worse, they are the small ones that come out in the late spring/early summer and bite.