You can get a replacement from Citroen but they will charge you an arm and a leg to code it all in.
Most simple and best thing to do is buy a Hawk central locking kit for £19.99 and fit it yourself, take no more than 20 minutes and you get the added bonus of your hazards flashing when you lock or unlock!
Here is the guide to fit it -
http://www.saxperience.com/forum/arc.../t-360742.html