Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Attach U-gauge to the inlet pressure test point on the gas valve
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

With the appliance running does the pressure gauge show and inlet pressure of at least 18mb or greater?
Note:
1. The appliance should still light with pressure as low as 16mb. Although the cause of any low pressure would need to be investigated.
2. When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.
3. Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.