Vokera HE Condensing Combi Boiler?

Got a relatively new vokera boiler - 18 months old and it has developed a problem over winter. Fault warning - flashing green LED 120hz comes on - manual says temporary fault. Then eventual RED comes on and the unit stops heating. I notice at ignition time the pressure is in the correct range 1-1.5 bar but it increases over 1.5 and shortly after the unit fails. Why is water pressuer increasing? Ive checked the crossfeed and its not leaking