Seems like your expansion vessel needs pumping up.
you need a footpump.Switch off the boiler.depressurise the heating.find the expansion vessel valve(same as a car tyre) at the back top of boiler and pump upto 0.5 bar represurise boiler to 1.5 bar.
Hope this helps.
Pete 100% Heating ,Bristol 01454854067, November 2006
