Set Appliance State:

Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Boiler Operating Switch in HW position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across connection 3 (Grey and Red wires) and the Neutral connection (Blue wire, marked N) on lower connector block.