Set Appliance State:
Mains Power Off
Set Multimeter to AC Volts 
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Boiler ON/OFF switch ON.
Measure voltage across the black wire on CN5 and the mains neutral connector on CN7.
Measure voltage across the black wire on CN5 and the mains neutral connector on CN7.
Isolate as per TOPs P4
and refit as necessary