Set Appliance State:

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

Allow burner to carry out ignition sequence.
Does the inlet pressure remain satisfactory throughout?
Note:
When "U" gauge is removed test point should be tightened and checked for gas tightness.