Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Pre-heat switch OFF
External controls (i.e. room thermostat/programmer) calling
CH switch ON
Gas On
Mains Power On

Measurement:

Measure voltage across connection marked D36 (yellow wire) on the 4 pin connector on PCB 23 and main neutral connection (blue wire) on main terminal block.