My overflow pipe has been leaking for a while now and last week I had a plumber round to have a look at this and he advised that this was because the pipes were clogged up with some black gunge and this was casing the tank to overflow and the system would need to be flushed out which would cost a few hundred pounds. I mentioned this to a few guys at work one of which is currently doing a plumbing course and after he talked me through how a system works the original plumbers explanation doesn't seem to make sense and it seems more likely it's an issue with the ball valve. Can anyone confirm weather the plumbers explanation seems possible or is it more likely a problem with the ball valve?