Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4

Observation:

Remove and inspect the metal Balance Tube that runs from the Modureg on the Gas Valve to the boiler inner air chamber for obstructions or signs of damage. Also inspect the balance tube 'O' ring seal at the Modureg and the balance tube air chamber grommet for signs of damage.
Replace Tube After test.