HABITATION DOOR

If it is the same as our 2012 Worcester and you can get the door open, carefully remove the handles and gently prise off the trim panels, they are on push in plastic clips and you will see the door mechanism inside. I am afraid I do not have a picture but the whole thing is only made of plastic and within it there is a spigot on which part of the lock rotates. There is nothing to stop this part falling off the spigot so if course it does!
The trick for us was to locate it and as recommended by Marquis put some glue on the end of the spigot then it can't fall off again. It is not an uncommon fault so that may be it.

Sorry that may not be very clear but basically I am saying if you carefully expose the lock you should be able to see what is wrong with it, not the greatest piece of engineering.

Alternatively if you would like me to do it for you just send a first class ( business class will do) ticket to NZ and I am happy to help or get Al to pop round on his next trip!! Good luck

It is probably a Seitz CaraD door. This PDF has exploded drawings of the door and ancillaries.

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