Hey buddy, you have exactly the same issue as me... I aquired a '99 P38 last week with only one key fob. (Expect to see me in the "Introduce Yourself" section imminently!)
I would NOT recommend a LR dealer.
I called the LR dealer near me, and was shocked at the absolute lack of interest on their part. Couldn't even quote me the price of the key ("Dunno mate, depends on what type of key it is, need to see it really, anywhere between 40 and 180 quid"). Pathetic. And of course "Then we need to code it, that'll be about 40 quid. Key could take anywhere between 5 days and 6 weeks to arrive". Shocking.
I then called a local LR parts shop, who quoted me £112 plus VAT, deliver by post to my door within 10 days. He advised me of a local garage who do rangeys and they have offered to code the key and do a full diagnostic for £50... they'll even wait back until I get there from work in the evening.
LR are truly shocking... if you can find a good independent parts seller, use them - and take their advice!
Try Dingocroft in Bucks, they seem to know everything. I previously had a classic EFI and they helped me with starter and reverser switch problems.
Jock