Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Set flow rate at tap to approximately 3 litres/minute
Attach a pressure gauge to the gas valve outlet pressure test point
Selector switch to DHW position
CH temperature selector to minimum
DHW Preheat selector to Off position
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow burner to ignite and DHW to heat to approximately 60°C.
Does the gas pressure decrease as the burner modulates?
Note:
When pressure gauge is removed pressure test point should be tightened and checked for gas soundness.