Bought a syphon valve from Wickes recently , and the threaded flush pipe that fits through the cistern is about 2 cm shorter than the old one.
I remember having similar problems with another one a few years ago and I replaced the old donut rubber seal with a new one and still had water coming...
I am in the process of fitting a new bath into a first floor flat I have only just bought. Tenant downstairs is saying that there have been 3 leaks when the previous tenent was there - the leaks are apparently coming from where the waste pipe in the kitchen goes into the soil stack , the soil...