A large gathering of flies in your loft space, attic or wall voids particularly in autumn is likely to be cluster flies. Common cluster flies (Polleniarudis) are large and black and look very similar to house flies so can easily be mistaken for them. If you seal cracks and crevices around windows, under eaves, fascia’s and door frames this can help to prevent them entering. Once cluster flies are inside you can use DIY direct contact sprays on visible adults, but for those flies already hidden and hibernating you will need the expertise of a professional pest controller to eliminate them completely.