Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove black plastic top cover from DHW microswitch
Disconnect one of the green leads from the microswitch, noting its position.
Remove black plastic top cover from DHW microswitch
Disconnect one of the green leads from the microswitch, noting its position.
Measurement:
Measure resistance across the terminal marked COM and the terminal marked NO on the microswitch, with DHW not running.
Resistance should be open circuit.
Turn on hot tap and repeat.
Test resistance should now be short circuit.
Turn off tap.
Replace lead after test.
Note:
This test will need the DHW to be turned ON and OFF.
If the hot tap is distant, then the tap can be opened, and the appliance water inlet service valve may be used to turn water on and off.
Resistance should be open circuit.
Turn on hot tap and repeat.
Test resistance should now be short circuit.
Turn off tap.
Replace lead after test.
Note:
This test will need the DHW to be turned ON and OFF.
If the hot tap is distant, then the tap can be opened, and the appliance water inlet service valve may be used to turn water on and off.
Isolate as per TOPs P4
and refit as necessary