Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Carefully remove wires from thermistor.
Carefully remove wires from 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 degree C.
Replace wires after test.
A reading of approximately 12Kohms will be obtained if boiler is cold.
This corresponds to a temperature of 20 degree C.
Replace wires after test.
Isolate as per TOPs P4
and refit as necessary