What is causing my houseflies?

i have been in my current home for 3 years, when i moved in there was a horrid smell about the place (council house) and loads of flies,(summertime) i hung fly papers bought repellent and fly spray which helped,(slightly) i bleached and disinfected the house from top to bottom which also helped mask the smell, the council came and sorted out the drains to try and stop the smell, the flies never entirely disappeared even during the winter when we have many days of freezing temperatures i still have about 20 very lethargic flies in my living room, i do a daily spray now and hoover up the dead, but im on the point of moving out so ive cleaned right through as im packing and i ventured into the attic for the very first time since ive been here there are patches of insulation just crammed with dead flies, loads of very slow flies up there i couldnt go up into the attic as it was way to dusty is anyone able to shed some light onto what maybe causing these flies the attic appears to b empty