First it implies there is a leak.
See http://britishgas.boiler-systems.co.uk/repressurise-systems/
Second there are two taps that need to be opened to let water in from the mains.
get help. Open the tap at the mains end and then open the tap on the heater pipe end a little until the pressure reaches 1.5 close taps in reverse order.
But get help
roger
December 2009