The headlining panel does, as Richard rightly says and I followed his advice, come out the tailgate. I had to remove all the boot area trim down to the wheelwell panel, and then gently bending the panel pulled it out.
Again, as Harv says, it is two thin sheets of fibreglass with foam sandwiched in between. I am sure that it is repairable using a car fibreglass kit. If you have experience of the old way of car repairing using fibreglass and Isopon then it should be easy enough.
What I would say is to take a lot of care if, and when, you repair the panel, and make sure that the downside-facing surface is absolutely smooth before you glue the material back on otherwise you will definitely see the edges afterwards. I have a small edge line showing beside one of the pull handles because, for some reason, the panel had a crack in it. The fibreglass wasn't actually broken but if you ran your fingers over the surface you could feel a slight difference.
I thought that this wouldn't be noticeable, as there wasn't a visible crack or split but you can see it if you look carefully.
The edges are a bit tricky, just try your very best to avoid the two fibreglass surfaces from seperating around the edge when cleaning off the old material. It comes apart very easily, and, unfortunately, the new material won't really pull it back together. The edges also need to be quite firm as they hold the roof panel in place by sitting in the door seals inner track.
My advice for the holes, for the grab handles, roof lights, and other bits is to cut an "X" in the material from the back surface, and make sure not to go too close to the edge of the hole, just to let the material pull through to be glued down on the back of the panel. I tried to be more "creative" with one such hole but it meant that the very corners didn't pull right through the hole onto the back. Fortunately, there is a reasonable edge around the fittings which will cover any small issues.
I found that the easiest way to get the rear reading lights back was to use one screw which was about twice the length of the original screw and let it pull the light fitting up to the plastic plugs in the roof. I think that the light fitting could break if you try pushing it too hard to get the original, short screws in place.
Also, the grab handles can take a fair bit of shoving to get them back in place. They have small lugs that push into a hole in the roof, but I found that I had the stick the screws onto a screwdriver and then, pulling the spring-loaded handle down, drive the screw into place. A bit a a pain but not too difficult.
The sunvisors are a bit of guesswork, but if you have cut a small hole for the mirror light cable then you should easily find one screwhole. Get one screw in, and you should be able to feel the hole through the roof panel by running your fingers across the material, and then push a screw through the material until you feel the screw hole in the roof.
I found that I had to bend the newly upholstered panel a little bit to get it back into the vehicle. But if one is gentle and wiggle it about a bit it goes in pretty easily. I think the best method of getting it into position without doing damage, especially if you like me have a sunroof, is to get the panel to sit in the rear passenger door seal track first. This will take the strain off the panel and give you time to get the front door seals done.
After that, refit all the pillar trim pieces. It should be easy enough to get a couple of replacement seat belt adjuster covers. I just made sure to wash [clean] the pillar trim pieces before refitting.