Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect the leads from the CH thermistor

Measurement:

Measure resistance across the exposed terminals of the thermistor.
The resistance measured will depend upon the temperature of the boiler.
The higher the temperature the higher the resistance.
The resistance should conform to the table below:
25 Deg C 1000 Ohms
60 Deg C 1300 Ohms
80 Deg C 1490 Ohms