Set Appliance State:

Mains Power Off
Isolate Mains Power to appliance
as per TOP's P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
DHW temperature control to maximum
CH temperature control to minimum
Remove plastic cover from the PCB

Measurement:

Allow the fan to run and the ignition sequence to commence.
Expect approx. 100V AC
Check for mains voltage at connector M10 on the PCB across the red and yellow leads.