Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Mains Isolator Switch (ON/OFF Switch) in ON position
Selector Switch in DHW Summer position
Remove wires from Microswitch and make safe (1)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Turn on DHW tap.
Measure resistance across main flow switch microswitch terminals.
Resistance measured should be short circuit across main flow switch microswitch.
Turn off DHW tap.
Measure resistance across main flow switch microswitch terminals.
Resistance measured should be open circuit across main flow switch microswitch.
Mains power OFF and replace leads.