my wife's office is in a basement of a Victorian building - it has a wc and a small hand basin. Every time there's a heavy shower, water comes shooting out of the waste, flooding the place. The basin has a P trap in 1 1/2" pipe. Its in Central London and I'm fairly sure that its a combined sewer. Any solution temporary or permanent? A stopper? A one way valve in the waste? Or get rid of the basin? Any opinion would be much appreciated.