Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove leads from dhw flow switch
Dhw flow switch is located bottom right hand side of boiler

Measurement:

Test 1:
With no hot water tap open, measure the resistance across the connections flow switch.
Expected resistance should be open circuit.
Test 2:
Open a hot water tap (minimum flow rate 2 litres/min), measure the resistance across the connections flow switch.
Expected resistance should be short circuit.