Cluster flies only really seem to be an issue in rural areas due to their breeding activity. Entering in the autumn in search for somewhere warm to hibernate for the winter, they gather in large numbers and are certainly considered a pest in the households they invade.
Because they lay their eggs in the soil, close to the habitats of the earthworms, in the summer months they are rarely a problem. However, when the flies come looking for shelter for the winter, often the only places available are the roof spaces of nearby houses. Due to their size, it is impossible to prevent the flies from entering as every house has tiny gaps between window frames and under fascias which allow them to get to the warmest spots in a property.
Until the first frost, they will continuously enter homes looking for shelter until the first frost and so generally over the winter months they aren’t much of a nuisance. People often won’t notice them until the spring when they wake from hibernation and attempt to return to the outdoors in order to continue the cycle.