contact your nearest land rover dealer. as long as it is the original radio, they should have a record. If they won't help, try the original supplying dealer (i.e. when it was new).
If you have grief, try ringing round a few dealers, there seems to be differing policies on giving out the number. Some ask you to fill out a form, some make a charge, some won't help, some just give it to you if you prove you own it (i.e. logbook & drivers licence).
A friend of mine (zafira) went to the supplying vauxhall dealer, who wasn't really interested, said it would take 3 days and he would have to fill in a form, prove his id and pay £35. Said he would be better taking it to a back street radio shop and have it decoded there. He then went to another branch of the same dealership. Not only did they give him the code there and then for free, even offered to go and punch the code in the radio for him. Top guys.
good luck