Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach pressure gauge to gas valve pressure inlet test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the burner to ignite and establish and the appliance to run in DHW mode.
Does the pressure gauge show an inlet pressure of at least 16mb or greater while the appliance operates?
Note.
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.