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

Observation:

Observe the U Gauge throughout the ignition cycle.
At the sparking stage of the cycle there should be a dip in the standing pressure indicating the gas valve has opened.
How does the inlet pressure behave?
Note:
When the test is complete the pressure test point should be tightened and checked for gas soundness.