The problem has been identified and now repaired.

As there were no posts, I guess no one else has this issue, have you checked to ensure your filler flap does actually locks? :chairfight:
The problem was that the actuator / solenoid locking pin was not coming through the sleeve and locking the flap.
The first picture shows the sleeve removed and the locking pin in the correct position when the car is locked.
The second picture shows the cause of the fault. The sleeve has four legs, one of which had fractured and caused the sleeve to distort, resulting in the locking pin not engaging through the hole.
The third picture shows a new sleeve (left) pictured against the faulty sleeve (right), showing the fractured leg.
