Broken door lock

I have a 57 plate Clio Campus. I like this model of Clio, easy to drive although not a lot of power. The only problem is that the single key operated lock (on the passenger door, the driver door on a left hand drive car) does not work. The central locking is fine, but I have no way of locking / unlocking if this goes belly up. It is seized. I squirted WD 40 into it and the whole mechanism turns through a whole turn rather than activating the locking / unlocking mechanism. I suspect that this doorlock has never been used by the previous owner in 6 years and it is corroded. How does the lock work? Can I put in a replacement mechanism or does it need to be coded to the car in some way? I just know that a dealer will want to replace everything they can at an enormous price which I am not willing to do.

Suggestions welcome before I get the spanners out!