Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
Selector Switch ON
Gas Control Knob in Main Burner position
Unplug connector CN1

Measurement:

Measure resistance across Modulation Coil (Grey wires).
This should be approximately 120 ohms.