Key Fob for 200 series Disco not workin

I have a 1992 200 Series tdi diesel Discovery, the Key Fob just decided that it doesn't want to work anymore. How can I reset it so the alarm doesn't go off every time I turn the Key? It won't start either as the immobiliser won't shut off!!!
I hope there's a simple method of doing this. Ohh the key fob is a single button one - actually I have 2 key fobs, the original one hasn't worked since I got it 4 yrs ago - presumably that's why there is a second one!

Cheers

First qu. is it standard alarm etc ? Ihavent used fob on my 200 for over a year, just key and alarm etc set fine. sugest power up shut off soleniod to make sure it is imob. prob or somthing else.

try this

Remote Programming - Single Button
Must be carried out in 8 seconds to access learn mode. Best done with two people. You should have both remotes available or second one will not work.

* Open hood and have you wife hold hood alarm switch down.
* From inside car with all doors closed, ignition off and all lock buttons up
* Turn ignition on then off
* Cycle drivers door button down then up
* Wife now releases hood switch
* Turn ignition on then off
* If done correctly, horn will beep once and dash LED will light up. This indicates the learn mode has been accessed. If this doesn’t happen repeat 1 thru 6.
* From inside car push and hold first remote. The dash LED should flash 8 cycles a second for one second indicating that the code was received. If this does not happen repeat steps 1 thru 6 again.

Push and hold the second remote if the code is received the dash LED will flash 8 cycles indicating that the code was received. If this does not happen repeat 1 thru 8 again. Note: Turning the ignition on at any time will cancel the learning process. The system can only store two remote codes and must be done at the same time. If you only have one remote program it twice.