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 the Thermistor

Measurement:

Check resistance of Thermistor is between 1Kohm and 14Kohms.
A reading of approximately 12Kohms will be obtained if boiler is cold.
This corresponds to a temperature of 20 deg C.
Note:
Replace wires after test.