I just fixed similar problem on my Ravenheat today. Pressure vessel needed repressurising. Bastard of a job to get to as my boiler has been built into an airing cupboard and the PV valve is right at the back, under the wall fixing bracket, up against the wall... much swearing and bruised knuckles before I finally managed to fit the bike pump! However... boiler now ticking along nicely, with no loss of pressure and no abnormally high pressure readings either.
Had a plumber out to look at it a while back. Told me I needed a new PV and that he'd have to rip out the airing cupboard to get to it. Thank God for Google!!
Mel
February 2010