Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Boiler On/Off Switch ON
Disconnect the Pump Flow Switch connector plug PL9 from the PCB
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the Resistance across the two connections (Brown wires) at the disconnected plug.
Note:
Measurement taken at the disconnected plug not PCB.