Local independent Land Rover specialist? Be cheaper than the main dealer but I think you ideally need a specialist on this if it is an off road motor as you say, you never know what has been messed about with, lots of electronics and all that jazz.
As @1988smithy has asked, do you have the EKA code? On the Td5 Disco 2, don't expect to find the intruder alarm/immobiliser, it's all built into the body control unit (BCU).
For more information about the immobiliser check out @neilly's excellent post at:- https://www.landyzone.co.uk/land-rover/handy-buying-alarm-info.311291/
Particularly the attachment "D2 Alarm issues rev5.pdf"