Set Appliance State:

Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Gain access to terminal strip
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

With a Central Heating demand, measure the Mains Voltage across connection 1 (Violet) and N (Blue) on the Appliance Terminal Block.