Set Appliance State:

On/Off Switch 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
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Appliance On/Off control set to On position.
Measure the voltage at connector ST1 centre pin (white wire) and a suitable neutral.