Although this appears to be a rare problem, I'll tell you how I went about it... Quite involved, not for the faint hearted. I removed the side trims and the aluminium rear screen surround. Released the headlining, having marked many reference points for gluing it back in, on the headlining and interior panels, to make sure it went back in the same place. I then removed the bows working from back towards front.
The bows are of spring steel but mine didn't have much strength and were easy to distort. After some consideration I decided to bond some carbon fibre strips onto the bows, which has made them much stronger. Luckily there was a strip available measuring 11 x 1.8mm. I bonded it to the bows using PU18, after keying both surfaces. Then reinstalled from front to back. The reference markings were essential to get it back in the right position. The long side trim pieces also help to do some final tensioning of the headlining. All seems to have worked successfully and back in with no creases. I haven't road tested it yet but I am confident!!!
Thank you for the advice, particularly William!
