Central locking not working

Wel lads, i have central locking (key in door) in my e10 saloon.

The drivers lock unlocks all the doors but only locks itself and the rest have to be manually locked.

It was broke before i got the car so i dont know when or why it stoped working. The front outside passenger handle has been changed and there is no lock there any more. Could it be just this thats causing it not to lock the doors, or is there a more common problem (fuse/relay etc...)

I havent had a chance to take the door apart and see for myself and I wont be able to have a look untill tomorrow evening, but a heads up on what common problems are is always nice