if you are 100% sure the filling loop isnt passing and the pressure is rising even when the boiler is cold, the most probable cause is the domestic hot wtr heat exchanger, manu facture no 248048. there is probably a pin hole between the plates of the heat exchanger allowing mains water into the central heating side. order the the rubber washers at the same time as they will need to be changed.
P D
August 2007