Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Gain access to lower PCB panel
Ensure Central Heating Turned OFF
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage between CH on the yellow connector (marked LR with a radiator a symbol) and mains neutral.
Mains voltage should be absent.