I have the same Range Rover you do. When you change the battery you have to keep constant voltage to the leads you take off the old battery or you have to enter codes into your vehicle. A reat pain in the ass, but I did it myself. I think your owners manual has the codes in it.
I wrapped some 16 guage wire leads from the battery leads in the RR, to the posts of the battery I was removing. Negative post to negative lead, positive post to positive lead. Once I got the old battery out without losing contact, I dropped in an Optima 800 red top and put the leads on the new battery and removed the wires from them.
Range Rover
Land Rover DII
2 Toyota Land Cruisers