Check that the orange expansion vessel is working correctly. Undo Schraeder valve (bike pump nipple) and move needle asside briefly, is air let out or water? Water? get a new expansion vessel. Air? Check pressure with car pressure guage. To do this, let all pressure out of heating system darin down until pressure guage on boiler reads 0, stop there. What is pressure at Schraeder valve? 0.5 to 1 bar? OK Less? Pump it up with a bike pump.

Refill, get heating water jolly hot, turn off and blow it all out of the prv. The hot water jetting out may just clean the seat, but probably won't. If it doesn't work, buy a new prv. To fit, drain boiler (through prv) after isolating flow and return pipes, fit new prv, re-open valves and top up.

Job's a goodun.

Great combi by the way, wish Worcester were still that good.