Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect orange wires from overheat thermostat
Disconnect orange wires from overheat thermostat
Measurement:
Measure resistance across spade terminals on the overheat thermostat.
Resistance measured should be short circuit.
Note:
Overheat thermostat is of the auto reset type and should only be open circuit if boiler is at excessive temperature i.e. above 110 degrees C.
Resistance measured should be short circuit.
Note:
Overheat thermostat is of the auto reset type and should only be open circuit if boiler is at excessive temperature i.e. above 110 degrees C.
Isolate as per TOPs P4
and refit as necessary