Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Attach pressure gauge to outlet pressure test point on gas valve
The outlet pressure test point is the right hand of the two ports
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow ignition sequence to commence.
With DHW demand does the does the pressure gauge show the following sequence :
STATEPRESSURE
Fan off0 mb
To ignition 3 mb
Burner establish 0 mb

Note:
When pressure gauge is removed pressure test point should be tightened & checked for gas soundness.