Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove lower facia panel
Disconnect the connector ST16 situated at the bottom R/H side of the PCB
Disconnect flame sense electrode lead at the inline connector

Measurement:

Measure the continuity (resistance) of the white lead near the centre of the connector ST16 to the inline connector plug.