try switching the boiler off for roughly 10 secs then switch back on, try to re-pressurise again using the filling loop - using blue extension tap to the right underside, you should hear water flowing and the gauge should increase, switch off at around 1.0 bar.If this does not work you may need a new valve as these do commonly break and stick shut, the valve is plastic rubbish - metal ones are available I believe.
I am assuming your only problem is getting the boiler to pressurise and not constantly losing pressure, if this is the case the likelihood is that the PRV requires descaling or replacing.
CAUTION - do not overtighten the blue tap as it is very easy to break the black plastic surround that the blue extension tap attaches to.
Good luck!
Rob, January 2010