If you see flies in winter, often in large numbers, then they will most probably be Cluster Flies (Pollenia rudis). They are similar in size to, and often mistaken for, Common House Flies, although they are noticeably slower in flight.
Typically Cluster Flies will find warm places to over-winter. It is not unusual for people to go to their attic in search of Christmas decorations, only to be confronted by a swarm of flies covering an entire wall (Cluster Flies are also known as Attic Flies). They also congregate around windows and doorframes.
They breed outside and then seek refuge when the weather turns cold. They can appear any time from late autumn to early spring. They often enter the house on the warm (South-facing) side.
Cluster Flies are a nuisance but they do not spread disease to the extent that other flies do. Treatment will depend on the location. In attics, insecticidal smoke generators are effective. Residual sprays and powders can also be used to good effect.
If you have a problem with Cluster Flies you can call Ged