Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove both ends of the flame sense harness, from the electrode and from X2B on the PCB at the back of the lowered facia
Remove both ends of the flame sense harness, from the electrode and from X2B on the PCB at the back of the lowered facia
Measurement:
Measure the resistance of the flame sense harness lead.
Isolate as per TOPs P4
and refit as necessary