Losing all pressure from system when central heating is switched off?

I have a Vokera Option Boiler, which from what i have heard is one of the worst boilers to have!!! Anyway, i have a similar problem as the one described in the related question, where i am losing the 1.5 bar pressure when i turn the boiler off, i therefore have to continually fill the system from the filling valve in order for the boiler to work. Looks as if i am getting a constant dripping from the prv outlet outside, can anybody help, what maybe the problem?? And what should i do??

Kendo 799
December 2008