How to fix pressure problem on Ferolli F30 Combi?

I have a Ferolli F30 Combi which has for a while now had a problem with pressure.

It will only work when the pressure is filled to 3.2 Bar (pre-firing) for both heating and hot water. Hence at some stage this week the prv has kicked in and released all the pressure for a day, but now is back running when above 3.2 bar again.

I have checked the expansion vessel and only air is coming out (which is a good thing). I have lowered the pressure to between 1 and 1.5 bar, as this is where it should be working.

The system fires up for a couple of seconds for heating or hot water, but than shuts down with the "low pressure" light flashing.

I don't know if I can fix this my self or whether I need someone in? Could it be a suspect "low water pressure switch"?