Ok Alex. The immobiliser ECU is behind centre of dash, communicates with the chip in the key, the one that you put in the lock, and is a complete dash removal to get at.
The fob, the thing with 2 buttons on, talks to the CCU and can easily be reprogrammed to match the correct but uncoded or incorrectly coded fob. If you buy a fob and a CCU then they talk to each other and the CCU is behind the inside fuse box. Make sure, if you change the CCU that you get the correct 3 or 5 door one to suit your vehicle.
If you have no actual key to copy, only LR can produce it unless you change the locks and key and then the chip in the key head portion must match the Immob unit.
Good luck !!
Some people call the fob a key and get confused, that is why I have spelt it out !!!