Door Locks, but not Open - which part?

My 2001 TD4

Nearside-rear passenger door will lock under use of the central locking, but the central locking doesn't open the lock, needing a manual reach-around to pull the door latch to open this door.

I can hear a dull clicking noise when I try to open this door with the central locking, but the little indicator tab doesn't jump up.

Given it 'half works' (close but not open) can anyone suggest if this may be a repair project or full replacement job?

t.i.a.

Common enough problem that most often require a replacement door lock mechanism due to weakening/failure of the internal solenoids.

Since you can hear it trying to unlock you might be lucky and to get it to improve if you lubricate any visible moving parts. Failing that it's a new one I'm afraid

Ta very much.