Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Boiler Thermostat set to its '0' position
HW Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the HW external control connection marked SWL (L) (Brown) and the Neutral marked MAINS (N) (Blue) on the main connector block.