Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
DHW Demand
Attach U Gauge to inlet pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the inlet pressure with the appliance running and the burner lit.
Isolate the cold water inlet to the appliance using the service valve on the underside of the appliance while observing the U-Gauge.
The U Gauge should return to standing pressure as soon as the isolation valve is turned off and the demand ceases.
Note:
When "U" gauge is removed the test point should be tightened and checked for gas tightness