Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
Isolate Mains Power to appliance as per TOPs P4
DHW Demand
Gain access to appliance terminal block connections
Boiler on/off switch ON
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across connection L2 and N (neutral)?
Note:
If no external controls are fitted a link must be fitted between connections L1 and L2.