One door won't look via central locking

I recently bought a 2nd hand Focus, X Reg Ghia. There's a few niggles with it, but nothing I can't fix except the rear offside door won't lock when using the central locking. I can manually lock it from the inside, but if I use the central locking only 3 doors and the boot will lock, leaving this door vulnerable.

Any ideas what to look for when I take the door trim off and have a look?

Thanks in advance.