Sliding door on 03 Primaster won't quite close properly?

The sliding load door on my Primaster doesn't close properly, sometimes causing the alarm to sound, often causing the door open symbol on the dash to light up (-disconcerting, on the motorway). I can see that the contact plate on the door frame is only being struck at the lowest and furthest out point. The shut line to the front of the door is about 12mm. How easily can this be remedied? Wondering if i need to purchase new tracks and locators etc.