Set Appliance State:

Mains Power Off
Gas Off
Set Boiler Control Thermostat to MAX position
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
External controls calling for heat
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across live connection (red wire) at the neutral connection (blue wire) at the fan.
Expected voltage 230 V AC.