The 15mm pipe that goes into the bottom of my toilet cistern keeps leaking. The part that's leaking is where the metal fitting screws onto the plastic pipe that comes from the toilet. I have just renewed the washer that seats in the fitting and tightened up, turned on the mains and it's still leaking. I didn't wrap ptfe tape on the thread cause homebase didn't have none but didn't think I'd need it with a new washer. Any suggestion would be good. thanks