55 Reg polo - doors dropped - help!

I have a new model 3 door polo and I've noticed a patch of rust appearing on my drivers side door sill at the back and on the door in the same corner (its also started to scuff in the same place on the passenger side). When I took it to the dealership they told me i must have trapped my seatbelt in the door and that had caused the damage - I have never done this. They then took it to the body shop who said the doors had moved but that i must have let them catch in the wind so the problem wont be covered by my warrantee - I've never done this either!

I have taken it to another garage to get a second opinion and they have said that the doors have dropped and are catching the sill when they shut. There is no evidence of driver misuse as was stated by the dealer.

Has anybody else had this problem or advise on the best way to put my case to Volkswagen as they dont want to accept responsibility?