Under the boiler on the pipework there should be a valve
with a small lever on(known as a lever valve) fitted too the
cold water supply into the boiler,also on the central heating return pipe there should be a non return valve,between these two fittings a 1/2" braided hose is fitted to facilitate filling of the central heating system,this is done by fitting the braided hose and by opening the lever valve water will be passed from the cold mains water into the central heating.stop filling and re-vent the heating system when the pressure guage reaches approx 1.1/2 bar.after venting check the pressure and top up too approx 1.1/2bar.
tim
December 2007