Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
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

Measurement:

Allow the burner to ignite and establish and while the appliance operates monitor the pressure on the U gauge.
Does the inlet pressure remain satisfactory (18mb or greater) while the appliance operates?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.