how can i fix my central locking on ford mondeo 1999 td?

when i unlock drivers door with key it wont open the other 3 doors but when i unlock from front passenger door the central locking works fine. can you tell wat part might be worn or broken and maybe needing replaced, thanx ps the remote central locking fob/key is broken visibly inside i just thought i'd mention it. also i was thinking of buying a 2nd hand key of ebay is there complications i have not thought of concerning the immobiliser, they say they will give full instructions in easy steps but i'm wary if it would be so easy as i've heard stories of over £100 to programme similar things with other makes. thanx