I bought a 2004 4.2 sport a couple of years ago and it only came with one key. Local Jag dealer priced a new one at £300 ish which I was not prepared to pay.
I got a used one off Ebay and got a new blade cut (£12). I manged to program the fob to open the doors and work the alarm. The transponder is in the key part and I although it brought all the dash lights on it would not crank the engine.
A local locksmith tried and he could not program it either, he actually broke the part that holds the transponder trying to get at it.
I bought a second new key & fob off Ebay (£16.99) and filed the flats on the key (I used to be a toolmaker / die sinker).
I then took the car and keys to Jagtech at Stockton on Tees. They had quoted me £35 to program a single key and they did both for £48, which I think is excellent price and service.
I had researched many articles on key X350 key programming on the internet and youtube and it seems they all seem to refer to programming just the fob (alarm and doors) and not the transponder which has to be matched to the car to crank and start the engine.