The adult cluster fly is about 6mm long with distinctive golden hairs on the thorax (chest area).
They live indoors and adults feed on nectar. They lay eggs in damp soil and rotting leaves and in the autumn the flies look for shelter and hibernate over winter - usually in big buildings,lofts, and sometimes invading the living area of a house.
They tend to invade in very large numbers , sometimes in many thousands and the bad news is they can return to the same property year after year.
Prevention is very difficult as where one fly can get in, so can many more.
A fly spray will help, but it will not penetrate into cracks where flies are hibernating. We either use smoke generators for quick 'knock down', but the preferred treatment is a powered fogging machine that will penetrate the flies hiding places.
Please note - where bats are present in lofts or buildings it is illegal to use insecticide and the pest controller and house holder have a legal duty to ensure that bats come to no harm.