Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Remove the Yellow spaded wires from the DHW Thermistor and measure the Resistance across the terminals on the Thermistor.
The resistance measured will depend on water temperature and should lie between 500 ohms and 20 k ohms. The higher the water temp the lower the resistance should be. Modulation of DHW should take place with resistance approximately between 2.5 /3 K ohms.
Reconnect wires after test.
The resistance measured will depend on water temperature and should lie between 500 ohms and 20 k ohms. The higher the water temp the lower the resistance should be. Modulation of DHW should take place with resistance approximately between 2.5 /3 K ohms.
Reconnect wires after test.
Isolate as per TOPs P4
and refit as necessary