Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to Resistance 
Turn Hot Water Tap On
Electrically disconnect the DHW flow switch leads (black wires) at the inline spade connectors on the flow switch lead
Measurement:
Test 1.
With hot tap fully open measure resistance across the disconnected leads running from the flow switch. Resistances measured should be short circuit.
Turn OFF hot tap.
Test 2.
Repeat above test. Resistance measured should be open circuit.
Reconnect leads after test.
With hot tap fully open measure resistance across the disconnected leads running from the flow switch. Resistances measured should be short circuit.
Turn OFF hot tap.
Test 2.
Repeat above test. Resistance measured should be open circuit.
Reconnect leads after test.
Isolate as per TOPs P4
and refit as necessary