I thought that this would help you fellow XJ40 Owners and that's why I'm posting some instructions on how to program your keyfobs .

1990-92 XJ40
1-If you have a security code make sure you know it
2-Disconnect the negative battery cable
3-Reconnect the negative battery cable and the security system should start with a beeping noise
4-Press each transmitter so likewise 5 times till you hear the confirmation tones
5-Then enter the radio code if your audio comes with the security code

1993-94 XJ40
1-Turn the ignition key to the "on" position without starting the engine
2-Turn security switch to "off" position which is located inside the center consolebox
3-Turn ignition key "off"
4-Turn security switch back to "on"
5-Cycle security switch "on" and "off" rapidly4 times within 5 seconds
6-Press each transmitter totally 5 times until you hear confirmation tones

 

It also helps if you walk sun-wise around the car while chanting "Oh, Lucas, prince of darkness."