Found this from a quick google...
By Vernon Stent.....
As the winter nears many insects become scarce. Adult house flies, for example, die off. Thank goodness for that I hear you say. Just remember though, that house fly eggs live on and are to be found in their larval or pupal stage hiding in some dung or in some rubbish somewhere. They may also be laid in warm attics or rafters. Flies are opportunistic and will lay their eggs anywhere that is a food source. For instance, if a mouse of bird has died in your attic, flies will use its rotting corpse as the perfect host for its young.
If the pupae or larvea were outside they would stay dormant through the cold weather. The trouble with lofts though, is that warm air tends to rise and keep them relatively warm. The eggs won't stay dormant for long. The warmth in the loft will result in continuous development until the young flies are finally hatched and, as is their nature, start to rove around looking for food.
They won't go far. In the winter, the only source of heat will be the house itself. They may stay trapped in the loft and might die off once all food sources have been exhausted. They may also die off if the house occupants go on vacation and turn off the heating. In many situations however, they will make their way into the living quarters of the house and start to do their nasty work of landing on and sampling various foodstuffs. The kitchen is one of their favourite destinations.