Orian Gemini Cabin Door Lock

Having just had a two day "shake down" in our Orian Gemini we have learnt a lot about the operation of the van but one thing I don't understand is the lock on the cabin door. I can lock it with key from the outside but there appears to be no method of locking the door from the inside. I had to go outside, lock the door with the key and then re-enter via the passenger door in the cab. I can't believe this is the only way to do this. Can anyone put me right?

The inside 'knob' should turn to the right to open and turn left (and stay left) to lock.

The locking catch is inder the door handle/catch, press it in or pull it out

Sorry, thought it would be one like this

Might have been me misunderstood Paul when he says Cabin door I assume he means cab door not the Habitation Door

Having re-read , I'm not sure which door we are talking about.

I thought I would post this in case anyone else has a similar issue. I had a small list of warranty jobs after purchasing the van and included the habitation door lock. On returning this week to check the repairs I enquired about the operation of the lock from the inside. A smile came across the guy from the workshop and he showed me how this works.
It is all about the EXTERNAL key position. The external lock must be returned to the horizontal position ( -- ), not the vertical ( | ) on removing the key. If the lock is in the horizontal position, you can lock the door from the inside. However if it is in the vertical position, it thinks you have left the key in the lock and you can't turn the internal knob to lock it. 
I think this was left out of the handover but can't be sure as sooooo much info was given. 
I hope this helps someone in a similar dilemma