Richard,
Where you live are you in close proximity to other people with garage door openers, remote gate openers, or other cars with remote entry sytems? A few years back Land Rover issued a bulletin to dealers detailing how vehicles left for a few days without starting would run their battery flat. It seems that there is a wide frequency band that the Range Rover monitors looking for the signal from your remote. This signal wakes up the BeCM (body electronic control module) and powers up the computers before you enter and start the car. The problem is this frequency range is also used for many garage door openers, gate openers, etc.
So what happens is if the BeCM is sees atransmission in that frequency band, it powers up. If the vehicle is not started, it will power down again (I think in about 10 minutes). It will repeat that for every signal it gets. Normally this is not a problem, because it would only happen rarely. But, I've had several people with this problem if they lived in a townhouse or apartment d (high density of electronic activity) or lived or worked near airports. I had a pilot who would park his RR at the airport for a few days and when he came back it would be dead. Not a very happy customer.
The fix to this problem is an update to the software in the BeCM, not a replacement. I hope they checked this out first, before they try to charge you for replacement. Now I will say that I have seen faulty BeCMs cause all kinds of problems. If this is all the problems you are having, I think I would just invest in a small trickle charger and a jump box. A lot cheaper than a BeCM replacement.
Stephen