I assume you have a combi boiler. Pressure should be 1.5 bar on the pressure gauge. usually under the boiler there should be a silver braided filling loop witha valve on one end.
open the valve and watch the gauge rise (try tapping
it !) to 1.5 bar. Then go around the radiators in turn and let the air out at the bleed nipple - going back
to the pressure gauge each time to check and re - pressure at 1.5 bar.
That should do it. If it persists come back on and I will tell more
Bob the PLumber
Bob Foster, April 2009