Actually I've traced it to the tundish up in the roof. This is cold water coming out and its a constant light stream.Not sure if it would be the main tank thats pushing this out or the expansion vessel. Are there any aspects of this that could be just maintenance based (i.e. done by me) or, do I need to call a plumber?
suspect you have an unvented cylinder and if so it is normally connected to a tundish which in turn is linked to the prv or temp relief one of which i leaking