Set Appliance State:

Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Gas Control Knob in main burner position
Attach a U Gauge to the Gas Valve outlet pressure point
Gas On; Pilot On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Unscrew the cover for the Modulation Adjustment on the Gas Valve.
With the DHW tap fully open, increase the gas valve Modulation Adjustment.
Does the gas pressure remain below 13.1mb?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.