Vokera 24 losing pressure?

I have a vokera 24 combi that seems to be losing pressure. I pressurise the system to 1.1bar and while the heating is off all is well, when the heating comes on the pressure rises to about 1.5 bar and the house warms well. But when the system starts to cool down the pressure drops, within two days it will have dropped to 0.5 bar.
I have checked the PRV and its not dripping, the expansion vessel is pressured to 0.8bar and when I re-pressurised it there seemed to be no water coming out the valve so think its ok.
I am not sure what can be causing this loss of pressure aside from a leak in the system itself. Any help greatly appreciated.