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OK, just to successfully close this off, the bolt had dropped out from the front leading edge of the roof section. It is one of a number of bolts along the edge holding the front of the interior trim panel. On checking, all the others were loose so I’ve had a bit of luck really. If it hadn’t dropped somewhere noticeable I’d never have known.
All tightened up now. Pic for future reference in case anybody needs. For the 1 min it takes to check, I’d recommend everyone have a check of these at some point.