Been trying to pair a "new" engine ECU to my project track car

The cluster is from a 1999 A4 TQS, engine ECU is from a 1998 A6.

I've paired they key up with the cluster just fine, IMMO light goes out, cluster seems happy.

However the ECU isnt playing ball. I've followed the rosstech instructions on pairing the ECU to the cluster: http://wiki.ross-tech.com/wiki/index.php/Immobilizer_II_ECU_Swapping

However it doesnt seem to work. VCDS reports no errors or other issues. But the ECU still says Engine start blocked by immo.

Any ideas?