Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Carefully unplug the leads from the Thermistor

Measurement:

Check resistance of thermistor is between 1-14 Kohms. A reading of approx 12 Kohms will be obtained if boiler is cold. This corresponds to a temperature of 20 degree C.
Replace leads after the test has been completed.