Set Appliance State:

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

Allow fan to run and ignition sequence to commence.
Does U-gauge show a constant pressure of at least 19 mbar (37 mbar for LPG) throughout the operation?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.