Locking problem F/N/S door

I noticed last night that the passenger door on my Superb Sportline isn't locking. If I press the lock button on the fob, or use the keyless locking on the door handle, the indicators will flash and the door mirrors will fold in and all other doors will lock apart from the passenger door which remains unlocked.

If I then open the door (with the keys away from me) the alarm will sound, but all the other doors remain locked.

The door doesn't show open on the maxi dot.

The lock mechanism in the door appears to move manually the same as the others. 

Any thoughts?

Matt