The fault is most likely the passenger door lock/motor assembly failing to respond to the control unit's command to lock. This failure causes the symptoms you describe. Confirmation of the diagnosis is provided by the fact that when you lock the passenger door yourself (overcoming or masking the fault) everything returns to normal. Action needed: replace passenger door/lock assembly (it's an integrated unit on year 2000 onwards Mk2 models)
Howard
December 2007