Seat Belt Jamming on Copen

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Seat belts on my Copen tend to jam or come out of the guide on the side of the seat. I briefly met a guy who works for Ford at Dagenham who said that they have a mod that will cure that but did not get the chance to ask him what it was. I can only think that this may be something to do with the Ford Street KA. Has anyone else had this problem? If so any comments or advice please?