Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
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)
CH switch OFF
Mains Power On

Measurement:

Pre-heat switch ON.
Allow fan to run.
Measure voltage across the NO terminal (yellow wire) on the air pressure switch and neutral (blue wire) on the main connector block.