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
Use Insulation Mat as required
Gain access to the PCB
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between X1 and X2 of the PCB.
Expect approximately 220V AC.