Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Preheat OFF
Boilers Heating Temperature Control to OFF position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the Switched Live Input (Main terminal strip, 2nd from left, black wire) and the Main Neutral on the main terminal strip.
Expect Mains Voltage.