2002/3 nuevo habitation door

Hi all.  This might be a daft question but we have had our nuevo for a while now but I have not managed to find a way to lock the habitation door when we are inside.  We have tended to stay on Club sites  that are quite safe but I would feel safer if  I could lock it.  I have gone through the handbook but cannot find anything. Any help appreciated

When you open with key on outside, make sure key slot is returned to vertical. Then on inside turn knob anticlockwise to lock from inside.

Thank you.  I am actually in the van and have tried that and the inside knob wont turn anticlockwise and never has as I that was what I expected it to do.  Ithere is obviously something wrong with the lock.  The key outside turns to almost verticle but not completely.  I have just sprayed it with wd but no 
joy.  Looks like a job for a professional.