Same problem last week mate.Key fob battery was dead ,there should be a move you do with the key that allows you to reset the immobiliser-
Inesrt the key into the drivers door and turn the key four times to the locked position,ensure the key returns to the centre each time (this tells the alarm system that you are about to enter a code to disarm the imobiliser)
Turn the key to the unlocked position 1 time
Turn the key to the locked position 5 times
Turn the key to the unlocked position 2 times
Turn the key to the locked position 5 times
If the vehicle is locked press the unlock button on the handset then unlock the vehicle using the key.
This worked for me I then took it to an auto electrician who replaced the for battery and re coded the key for £47. I hope this works mate