Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to AC Volts 
CH Demand
Pre-heat switch OFF
CH switch ON
CH switch ON
Gas On
Mains Power On
Measurement:
Allow fan to run and burner to ignite.
Measure voltage across connection D24 marked LOFAN (yellow wire) and connection D23 marked N (neutral, grey wire) on 12 pin connector towards top L/H corner of main PCB 23.
Note: Voltage reading will modulate, test is to ascertain whether mains voltage is switched for low fan operation.
Measure voltage across connection D24 marked LOFAN (yellow wire) and connection D23 marked N (neutral, grey wire) on 12 pin connector towards top L/H corner of main PCB 23.
Note: Voltage reading will modulate, test is to ascertain whether mains voltage is switched for low fan operation.
Isolate as per TOPs P4
and refit as necessary