Forgot to mention; I turned off the stoptap and drained all the water from the hot water system. Then I unscrewed the immersion heater from the tank and water started to flow from the opening. I quickly put the immersion heater back in and checked that both H & C taps were open. Tried again; water still there. Where could this water be coming from please? It's an exchanger-type with a coil inside which is fed by a gas back-boiler.
Des.