Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Ensure there is no demand for either CH or DHW
Set Multimeter to AC Volts Set meter to measure AC Volts
Gain access to the PCB
Disconnect connector X1 from the PCB
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Again measure the voltage between X1 and X2 of the PCB.
Is approximately 220V AC now present?