Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
Gain access to mains terminal strip, use isolation mat as required
Boiler Thermostat in its '0' position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the external controls connection marked MAINS SwL (Brown) and the Neutral marked MAINS N (Blue) on the main connector block.