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
Selector Switch to Hot Water and Heating
Ensure all external controls are calling for heat
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

On the incoming terminal block measure the voltage between the blue (neutral) to the furthest away black lead (see picture).
Expect mains voltage.