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
User display control set to HW and CH
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the ignition sequence to commence and burner to ignite.
Measure the voltage between the red wire on connection X13 and the neutral at the incoming mains terminal strip with the connector attached to the pcb.
Expect mains voltage.
Note:
It is recommended to ensure the boiler has definitely fired for CH demand before checking the voltage as the boiler incorporates a built in delay.