Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Preheat OFF
Gain access to PCB (use insulation mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure AC voltage across the brown (L) and blue (N) wires if the boiler main terminal strip.
Expect mains voltage.