Cluster Flies tend to pick a property and stick with it for a number of years! Their offspring will also return to the same property. They are harmless as they do not lay their eggs into decaying and rotting matter, they lay eggs into eartworms so disease risks are relatively low. They do, however, become a pain in the Autumn as they come in their 100's into your home to hibernate over the winter months. Their presence can be disturbing as they make their way into your loft space or wall voids. When you turn on your heating they may wake up and make their way down into your home via gaps around water pipes etc.
The pest control shop has a range of killer sprays and fumers to combat these intrusive insects.