Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect the flame sense earth lead from the earth post on the l/h side of the appliance
Disconnect the flame sense earth lead from the earth post on the l/h side of the appliance
Measurement:
Measure continuity across the flame sense electrode lead and the earth connection on the plug
(the purpose is to check for continuity on the PCB pins).
(the purpose is to check for continuity on the PCB pins).
Isolate as per TOPs P4
and refit as necessary