Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect the brown and blue wires from the DHW limit thermostat
Disconnect the brown and blue wires from the DHW limit thermostat
Measurement:
Measure resistance across the exposed terminals of the thermostat.
Reconnect leads after test.
Reconnect leads after test.
Isolate as per TOPs P4
and refit as necessary