I had a similar problem in that the range reduced progressively over time, it turned out due to leaky sunroofs. If you hold the remote right in the centre of the two front seats behind the sunroof and try it may work. Its a really common failure item on disco 2. Pentland hold the receivers in stock( they will need your chassis no, as there is two frequencies). Kev at Bankhead may have one as well.
It should still lock of the key though? and if it's starting ok, it's likely to be something else, you haven't tried adding a new fob or anything like that in Nanocom?
I have a brand new receiver if your really stuck, got it for mine just not fitted it yet.
