ECU from ATP
I got an ECU from ATP 'cos mine got water damage. Arse. Anyway, disconnect the immobiler before fitting the ECU, otherwise it 'learns' the immobiliser code and won't start without a code key.
ATP recommend that you use the red key the first time you start the car with the new ECU to avoid later problems, so I would personally leave your immobiliser disconnected. I did. Sshh, don't tell anyone! Anyways, central locking still works, but not infrared remote.
Cheers,
Jon