Non start, indicators flashing when try to key start

Ok problem is that it won’t start, it’s doing the flashing indicators and lock/unlock sound - seems like immobiliser issue?

I believe that's fairly 'classic' CANBUS fault symptoms and, having been down this road recently (still am, in fact) it can be rather tricky to diagnose.

Common culprits on your age car would be:
The PCM (engine ECU) or TCCM (Terrain Control Module) which are immediately behind the battery in the 'e-box' - connectors accumulate water and corrode; check for water damage here
The BCM (Body Control Module)/CJB (Central Junction Box) which is near the passenger A pillar - if you take the glovebox out you can see some of it. Again, water ingress causing wiring shorts/faults.

At either end of the CANBUS are the ABS ECU and Instrument Cluster - the instrument cluster is known to fail (dry solder joints) but it's not unheard of for the ABS ECU to die as well.

Are you handy with a multimeter at all? There are some simple(ish) checks you can do to see if the CANBUS is 'complete' and can see the terminating resistors at either end:

Ohm out between CAN-HI and CAN-LO on the diagnostic connector (pins 6 & 14) with the battery disconnected - this should read 60Ohms. 120Ohms indicates you can see only one of the two terminating resistors and the CANBUS is broken somewhere along its length, possibly the TCCM (as this is a 'through' module), ABS or cluster (as those contain the terminators).

Measure voltage between CAN-HI and ground and CAN-LO and ground with the battery connected and ignition on and you should see ~2.6V on CAN-HI and ~2.3V (if I remember right!) on CAN-LO, but that's a very 'rough' test, you really need an oscilloscope.

If you have a diag tool like GAP's IIDTool, see if there are any ECUs 'not responding' and read the fault codes, they'll also give pointers. 2014 BMW 530d Touring, 2006 BMW 650i, 2018 Mini Cooper S