Immobiliser On My 08 Fiesta 1.25

[CTRAIG78]

My 08 1.25 fiesta, I went out to start the car and the car was immobilised, I called the AA OUT AND HE SAID IT WAS CODE 1 1 , after a few checks he said battery and alternator were ok , then car started after 5 mins car cut out and was immobilised again. I towed car to local garage and in the meantime he has overridden immoboliser till after the holidays car is running fine.

Any help appreciated