Set Appliance State:

Gas Off
Mode selector switch to Off/Reset position
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Hot water temperature control set to maximum
Attach a U-gauge to the gas valve inlet pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Mode selector switch to DHW position.
Allow burner to ignite.
Observe U-gauge. Does the gas inlet pressure remain satisfactory throughout the ignition sequence?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.