Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Remove the White spaded wires from the CH Thermistor and measure the Resistance across the terminals on the Thermistor.
The resistance measured will depend on boiler temperature and should lie between 500 ohms and 20 kohms.
The higher the temperature the lower the resistance.
CH Burner modulation should take place with resistance approx between 800 ohms and 2 k ohms depending on temperature selected on CH Temp Selector (potentiometer).
Reconnect wires after test.
The resistance measured will depend on boiler temperature and should lie between 500 ohms and 20 kohms.
The higher the temperature the lower the resistance.
CH Burner modulation should take place with resistance approx between 800 ohms and 2 k ohms depending on temperature selected on CH Temp Selector (potentiometer).
Reconnect wires after test.
Isolate as per TOPs P4
and refit as necessary