Its that time of year when the cluster flies start to become a nuisance again. The adults live harmlessly out of doors through the summer, but start to enter buildings, usually lofts and roof spaces and also through gaps in windows, in the autumn to hibernate, quite often in large numbers. The adult fly lays its eggs on damp soil and vegetation, when they hatch the larvae bore into an earthworm and feed on it until it dies, at which time they bore back out and pupate in the soil.
To get the best results from treatments it is best to wait until there has been a hard frost as this will force most of the flies indoors, but if the infestation is very bad you may want to consider earlier treatment with a second treatment when it is colder.