Vivaro 06 immobiliser flashing after battery change - please help!

[CMurdock]

Hi all, need some help.

Bought a 06 vivaro couple of months ago and its been running perfectly fine apart from the battery being a bit on the old side as it was losing charge quickly so it was replaced (alternator was tested and no parasitic drain was found so put down to old battery).

When battery was replaced by AA, it activated the immobiliser (flashing, not steady). We were able to intermittently start the engine using the central locking trick with the key/dashboard button but now it won't deactivate at all. AA autolocksmiths were called out disconnected and reconnected the battery, cut us a new key (we only had one) and reprogrammed the old key. No joy.

An auto electrician was called out who retried everything that the AA did, plus tested all fuses (I'm not 100% sure all were tested, just the passenger side and under bonnet ones) and they ran diagnostics which flagged communication with the ecu. He recommended replacing the entire ecu kit with a second hand one off ebay and said that the code he gave needs to match to jncrease chance of success but even then its not guaranteed to work.

Currently, no complete kits are available with the code we need. Is it essential that is matches? Secondly, has anyone else been in the same situation and replaced the ecu kit to fix the immobiliser issue? From googling, it seems to be a common issue in these vans but everyone seems to have rectified their problem from simpler fixes.

Honestly wish I never bought this van! Currently thousands out of pocket as its sat on the drive unable to start and only been used a few times. It was fine before the battery change :(