So the rear RH pass door is not opening on the 2016 Fab MC. The door opening handles are dead both inside and out. I can hear and feel the lock actuators working when operating the central locking but the connection to the door handles is not being completed. I put a little pressure on the door outer to take any preload off the lock to see if that helped but no joy. 
 

Am I right in guessing the deadlock mech is stuck in the lock and the only way to it is to dismantle the door from inside the car?

 

Would it need a new lock mech or are they good for bench fettling?

Managed to answer my own question after all. The lock needed replacing as there is no way to open it or the door without breaking the casing open. It’s not repairable as it is a bonded sealed unit. 
 

The cover of the lock needs cracking open at the bottom to expose the release lever. Flick it up to open the door and replace the unit. 

Progress is allowed!

Well done you!