Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to AC Volts 
CH switch OFF
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Mains Power On
Measurement:
Pre-heat switch ON.
Measure voltage across NC terminal (violet wire) of air pressure switch and neutral (blue wire) on main connector block.
Measure voltage across NC terminal (violet wire) of air pressure switch and neutral (blue wire) on main connector block.
Isolate as per TOPs P4
and refit as necessary