Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On
Boiler On/Off switch ON
External controls calling

Measurement:

Measure the voltage across the black lead (switched live) and blue lead (neutral) at the connector at the top l/h corner of the PCB.