I recently bought a fly trap online from a company called Clusterbusters. It's a narrow container of finely powdered eggshells and I can recommed it to anyone who has a cluster fly problem. You stick the container to the window pane preferably, but mine is fastened to the window glazing bars due to the small size of the window panes, and the cluster flies just fall into the powder and can't get out. Normally my bathroom window sill and the bath tub would be littered with dead flies in the winter, but only a very occasional one seems to avoid the trap. Apparently the pheromones from the flies in the trap attract more flies to enter. It's not a cure, and certainly doesn't look pretty, but sees to me to be better than hoovering up yucky dead flies every couple of days.