FWIW...
The driver's side lock on my Jazz has been seized solid since I got it, the previous owner said the passenger lock was also seized and the only way to unlock the car was via the tailgate.
Upon inspection, I found I could unlock the car using the passenger side lock but then I noticed something else too.
Seems like if I unlock the car from the tailgate (as I might if I was, say, bunging shopping in the boot) and then lock it again using the door, the door lock WILL be tight.
If, OTOH, I unlock it and lock it using the door, the lock feels fine.
I'm wondering if, perhaps, the central locking mechanism doesn't move the lock mechanism quite far enough, somehow, so it takes more strain to operate a lock that's previously been moved by the central locking as opposed to a lock that's been turned manually?
Basically, if I lock the car from the same lock I unlock it, everything's smooth but if I lock it using a different lock to the one I used to unlock it, the lock will feel stiff.
I currently have bigger (engine-shaped) fish to fry but I've got a remote locking kit to fit and when I get around to that I'm going to pull all the locks apart, clean them up and see exactly what's wrong with them.