Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Gain access to PCB (use Insulation Mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the boiler to run in CH mode and burner to ignite.
Measure the AC voltage between the right hand terminal on connector X13 on the PCB and the earth at the incoming mains connector.
Expect approximately 240 VAC.
Is the voltage measured as expected?
Note:
Access for the multimeter probe on X13 is in from the bottom of the connector.