Here is a good used copy of the Haynes manual that covers rebuilding the Renault BV 367 4speed, BV 369 5 speed and can be used for the UN1 as it is bascially the same box and the repair process is virtually identical.
The section covers dismantling and rebuild with close up clear images.
Clear images showing the setting up of crownwheel and pinion mesh is also shown
The buy now button will let you buy it from Amaz.on as we do not have stock now.