C30 ECM/CEM/ECU Fault After Battery Change

[AlexH97]

Evening everyone, this is my first post here so I’ll try and keep it brief. Recently bought a 2008 C30 (2l Diesel R-Design), when I got it it ran fine although the battery was old and weak, long story short the battery eventually died completely so I’ve replaced it with a brand new one, however this seems to have triggered the immobiliser fault which looking over the forums seems all too common! No crank, nothing at all, the key unlocks and locks the car, removes the steering lock and activates all the power but nothing else, when trying to start it pops up with ‘immobiliser try start again’ and I can hear a relay clicking somewhere but can’t pinpoint where to be honest. Fault C326 is stored in the fault log on my OBD scanner. Checked all fuses, connections etc so all good there, so I assumed the CEM and ECU had lost sync due to the battery issue. Found a company who can re-sync them so they’ve been taken off the car and supposedly re-synced but I’ve had them back and guess what, no start still and the C326 fault is still stored🤦🏻. Live data on the scan tool shows that the key is approved, start is approved but there is an issue with the ECM signal now apparently which is '3A00 ECM Software Faulty'. The parts have been sent back again to try and rectify this issue and have once again been returned but the issue is still there and the faults are still stored. All of the connections have been checked and cleaned, and I've now traced almost every ground in the car in case it's any sort of issue regarding that or voltage. Really at a loose end with this, can anyone suggest anything else I could try?

Thanks in advance!

p.s I’ve tried everything else I’ve seen on the forums such as locking and unlocking, leaving the battery off overnight etc and none of it has worked!