Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Slacken the lower test point screw of the Gas Valve and attach a U-gauge to the test point
Selector switch to DHW position
CH temperature selector to minimum
DHW temperature selector to maximum
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to start and ignition sequence to run.
Observe U-gauge. During the sparking phase the inlet pressure should drop slightly (by 1 or 2 mbar) then stabilise and remain constant at approximately 20 mbar.
Note: When U-gauge is removed the test point should be tightened and checked for gas soundness.