Depends what model Disco it is.... on my 300 tdi it was a different procedure to re-program your key fob.
What you have to do is put the vevicle into program mode
by.....
You Start with the doors shut the drivers window open the key in the Ignition and the Bonnet up but you need weight on the bonnet switch or a second person holding it down (But I use a tin of paint)
Now the next bit has to be done in ten seconds or you will have to start again.
Switch ignition on then off, lock and unlock the doors (with the pop up thingy), release bonnet switch, lock and unlock the doors again (with the pop up thingy), Switch ignition on then off again.
The horn should now have blipped once and the red indicator light on the dash should have come on.
If not start again
Press and hold the key fob button (for about ten seconds) until the dash red light flashes.
Then if you have another key fob press and hold that until the red dash light goes out.
If you only have one key fob you have to do the same one twice to make the light go off.
Job Done.
It sounds complicated but it isn't.
The stealers charge about £30 to do it for you.