Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Boiler switch in ON position
Central heating switch in OFF (central) position
Attach pressure gauge to inlet pressure test point on gas valve
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Observe pressure gauge, does the inlet pressure remain constant at a minimum of 20mb?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.