low pressure?

I have a glow worm compact 100e and has been doing the same problem over the last few weeks. The water tho has been dripping out of the PRV (pressure relief valve) and loosing pressure when the heating is on and goes down to about 2bar then stops dripping, then when its stone cold again the pressure is about 5bar. It turns out that the expansion vessel needs re-pressurising. maybe the same with yours?
Billybob.

Billybob
January 2011