low pressure in vokera linea he?

Hello, can I ask some advice? I have the above combi & a couple of days ago it began faulting on code 40, low pressure, after being off for the night. I checked the expansion vessel which was at 3/4 bar. Using the schrader valve I discharged it & no water came out. I drained the water system & recharged the vessel to 1 bar. Rechecked the pressure twice in the following hour & it hadn't dropped so refilled the water & fired it up. Within 10 minutes the pressure was 3.5 bar & coming out of the relief valve. The next time it cooled down, owing to the lost water, it was 0.4 bar & faulting. The flexible hose connecting to the vessel isn't kinked & the radiators are nice & hot. Could you please point me in the right direction? Thanks in advance for your replies