Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Gain access to the mains connectors on the PCB
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between L and N on the white leftmost connector of the boiler's mains inlet connectors.
Mains voltage expected.