we have the most peculiar problem in a flat. Our flat is a main door flat on the ground floor and there is an intermittent problem with an overflow pipe that is just below a second floor flat window. Every once in a while water from the overflow floods out with a quantity of water like a rising main which falls on the ground just at my front door. Inspections in the second floor flats have revealed nothing and none of the occupants can hear any water escaping from eg a cistern. Any suggestions on how to trace this fault please?