Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the inline connector to the DHW flow switch

Measurement:

Test 1:
Measure the resistance between the 2 pins on the connector the DHW flow switch.
Expect open circuit / O.L
Test 2:
Run the hot water tap with a flow rate of 2 l/min.
Measure the resistance between the 2 pins on the connector to the DHW flow switch.
Expect continuity / zero resistance.
Are the resistances measured as expected?