Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Loosen the screw of the P-Out test point on the gas valve
Attach a pressure gauge to the test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to enter its ignition sequence.
During the sparking phase of the sequence the fan pressure should be approximately 1.5mb.
Once the burner has lit, the fan speed should briefly decrease (approximately 0.5mb) then increase giving a pressure of approximately 2.5mb
As the boiler temperature rises the fan speed and therefore the pressure will gradually reduce.
After this test remove the gauge and make sure all disturbed gas connections are checked for tightness.