Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Unplug connector CN4 from the Main Control PCB
Unplug connector CN4 from the Main Control PCB
Measurement:
Ensure the HW Tap is Off.
Check for open circuit resistance across the DHW Flow Switch (both Red wires on the unplugged connector CN4).
Turn the HW Tap On.
Check for short circuit resistance across the DHW Flow Switch (both Red wires on the unplugged connector CN4).
Check for open circuit resistance across the DHW Flow Switch (both Red wires on the unplugged connector CN4).
Turn the HW Tap On.
Check for short circuit resistance across the DHW Flow Switch (both Red wires on the unplugged connector CN4).
Isolate as per TOPs P4
and refit as necessary