Set Appliance State:
Gas Off
Set Multimeter to Continuity 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Rotary Switch in DHW position
Keep Boiler Case Switch depressed
Rotary Switch in DHW position
Keep Boiler Case Switch depressed
Measurement:
Disconnect the two White wires that run to the air pressure switch.
One runs from the heating limit thermostat, the other runs from an inline spade connector.
Connect the multimeter across the disconnected wires (ensure the wires run up to the air pressure switch).
Mains power on.
With the fan running at high speed listen for a continuous tone.
Mains power OFF
Reconnect wires after test.
One runs from the heating limit thermostat, the other runs from an inline spade connector.
Connect the multimeter across the disconnected wires (ensure the wires run up to the air pressure switch).
Mains power on.
With the fan running at high speed listen for a continuous tone.
Mains power OFF
Reconnect wires after test.
Isolate as per TOPs P4
and refit as necessary