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:
Measure Mains Voltage across the Live (L) and the Neutral (N) connections on the 12 way Terminal Block.
If the boiler has an EMC filter (1) then voltage should be measured across the Live and Neutral connections on the input side of the filter.
If the boiler has an EMC filter (1) then voltage should be measured across the Live and Neutral connections on the input side of the filter.
Isolate as per TOPs P4
and refit as necessary