Superb estate rear door lock

I am having problems with the rear offside door lock on our 2018 Super 2.0L SE Technology diesel estate. The drivers door switch panel includes the two child lock buttons. When locking the car the rear offside door remains unlocked, therefore we have to keep it locked via the small turn screw on the edge of the door. Having looked into new lock prices, I decided to try a second hand one from a 'reputable' supplier, only to find that this replacement is doing the same thing. I conclude therefore that either I'm unlucky and the second hand unit is faulty too or there's a problem with the wiring. Does anyone have access to wiring diagrams for the locking system, or any suggestions as to how to test the locks? Thanks.