Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Apply a CH demand, making sure stats are turned up
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the voltage between mains neutral and the switched live input of the PCB X17.
Expect mains voltage.