Disco 3 (LR3) Reset immobiliser for replacement engine ECU

Hi everyone,

I have a 54-plate petrol Disco 3. I had a problem where all the cluster lamps lit up and the engine wouldn't crank, which I eventually tracked down to the engine ecu being dead.
In the Disco 3, it sits in a box (together with the transfer case ecu) behind the battery tray, at the back right of the engine bay. The box had filled up with rain water, and the engine ecu was sitting in the water rusting away. Eventually, the engine ecu must have given up and died, which I found after investigating everything else, like fuses, wiring, etc.

Luckily I found a replacement with the same part number from the US (Denso prt no. NNN500273).
Fortunately there is no corrosion on the connectors. I've replaced the dead engine ecu, and the cluster has all returned to normal, except one problem.. the immobiliser lamp is still flashing, hence the engine won't crank. I know there is some communication between the key/barrel, and the immobiliser coil to the engine ecu, but I can't find any info on how to reset the immobiliser. I figure it must be a reconfiguration in the engine ecu, to associate it to the key/barrel.

Any ideas anyone has how I can fix this would be really appreciated. Thanks for any help

PS, not a bad idea to check that box doesn't have water in it, and dry it out if it does. I've also drilled some drain holes in the bottom of mine.

The replacement ECU needs to be correctly coded to the rest of the vehicle modules where the chassis number is stored.
LR say you can't use a second hand ECM, but I've proved them wrong on my FL2.
However you will require some pretty decent diagnostic equipment like JLD SDD, or possibly a Gap Diagnostics IID BT, although I don't know if the latter can do an ECM swap.