Is it a combi boiler, if so it could be water from the cold mains leaking into the heating system the best way to tell is to do what you have done before by emptying the tank, underneath the boiler there should be an isolator for the cold mains input to the boiler turn this off and see if it stops the problem if it does it sounds like it could be the heat exchanger that is faulty
toptrish
July 2012