gavinh low voltage can cause dramas with the immobilizer. There's multiple checks, if you have Vida/dice you can see which module is blocking the start process (steering column lock, Cem Ecu)
Usually the steering column lock is the culprit for immobilizer faults on p1.
Sill question but you have locked/unlocked the car a few times with the good battery?
Excessive voltage drop when starting can cause it too
Sometimes the early p1s can lose key programming when the battery goes completely flat
