If, you're talking about the plastic sheet that goes between the roof lining and the underside of the roof, I suspect that your lining has 'dropped', as mine did last year. I worried about how to re-glue it back on without getting the spray glue everywhere. I've decided that a safer option is to use double-sided carpet tape (and not bother with the plastic sheeting). I've bought it (the tape) but haven't got round to doing it yet.
thanks, but thats not quite the piece that i an poorly tying to describe!! my part runs from the rear edge of the roof skin to the a pillar at the bottom of the windscreen area. it is about 6mm thick and 7' long.