Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Carefully remove wires from Thermistor

Measurement:

Check resistance of thermistor is between 1K and 30Kohms.
A reading of approx 12Kohms will be obtained if boiler is cold.
This corresponds to a temperature of 20 degree C.
Carefully replace leads after test.