Set Appliance State:

Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Selector switch in HW position
Gain access to PCB as required
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Mains Voltage across Domestic Hot water Thermostat (Red/Blue) and N (Blue) on the Appliance Terminal Block.