Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Central Heating Switch in Constant position
Room Thermostat calling for heat
Gas On; Pilot On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Is there mains voltage present on PL1 connection 1 (red) and neutral (blue)?
Ensure link is present if no room thermostat is fitted.