During the winter months you can encounter loft flies, properly known as cluster flies.
Types Flies in the UK
Cluster flies (Pollenia spp.) with golden hairs on thorax, body dull, tan to brownish black.
House (Musca domestica) and flesh (Sarcophaga spp.) flies with body dull, gray and black, thorax with 4 or 3 dorsal black stripes respectively.
Bluebottle flies, Calliphora terraenovae Macquart, C. vicina Robineau-Desvoidy, C. vomitoria (Linnaeus), etc. Adults about 1/4-9/16″ (6-14 mm) long; thorax dull, abdomen shiny metallic blue, lower squama/calypter (posterior basal wing lobes, posterior lobe) mostly dark; scutellum with 4-5 pairs of marginal bristles; found throughout the Uk
Blow fly disease carrying possibilities are often overlooked. Because many species feed on filth such as human excrement and sewage and/or develop in the carcasses of infect- ed animals, these flies may easily infect the food humans eat. Disease organisms may be mechanically transferred via external body surfaces, by their infected fluids during frequent regurgitation, and/or by infected fecal deposits. The list of diseases associated with intestinal tract problems is nearly identical to that for the house fly, with some of the better known including Escherichia coli (Migula) and Shigella dysenteriae (Shiga) which cause diarrhea, and Vibrio comma (Schroeter) which causes cholera. Non- intestinal disease organisms include plague (Pasteurella pestis (Lehmann & Neumann)), anthrax (Bacillus anthracis Cohn), tuberculosis (Mycobacterium tuberculosis (Zopf)), and tularemia (Pasteurella tularensis (McCoy & Chapin)).
Most species develop in meat or animal carcasses, but if these are not available they will use animal excrement, decaying vegetation, and/or household rubbish . Dead rodents, birds, and other small animals can be the source of flies within structures while dog excrement and household rubbish are common outdoor sources.
These flies are usually the first insects to arrive and infest after an animal dies. Their larvae are often used by forensic entomologists to help determine the time of death in murder cases.
Some species are strong fliers. For example, marked and released black blowflies (P. regina) had 13% trapped between 4-28 miles (6.4-46 km) from the point of origin and 8+% at or beyond 11 miles (18 km). Blow flies are most active on warm, sunny days, and primarily rest on cool and/or cloudy days. Inside, they are attracted to the bright light coming through windows.