Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Carefully disconnect Thermistor leads

Measurement:

Check that resistance of thermistor is between 1-30Kohms. A reading of approx 12Kohms will be obtained if boiler is cold. This corresponds to a temperature of 20 degree C.
Reconnect Thermistor leads after the test.