Set Appliance State:

Mains Power Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Ensure boiler is cool

Measurement:

Remove the wire from the thermistor and measure the resistance across the terminal on the thermistor and the boiler chassis.
The resistance measured will depend on boiler temperature and should equal to or greater than 800ohms.
Replace lead after test.