It is possible but you need to unpick some spot welding
So, I've now had the roof lining out and had a good poke around.
The rear mount captive nuts are already in the roof frame (I can see the rear most nut so am assuming the forward nut of the rear mount is also there)
The front mount requires a plate, pictured in my previous post. To fit this you would need to unpick some spot welds at the top of the front doors, slide the plate in, positioning it with the little screw hidden under the roof rubber, then spot weld that in place and repaint anywhere not hidden by roof lining or rubber.
You can't as far as I can see, screw a bolt in from underneath to show where the rear nuts are enabling you tp then drill down into the roof skin as mentioned in a old post on this forum so I reckon you locate the front ones which should give you the rear positioning.
I'm no welder so I'm off to ask a mate how much work would be required to sort the front mounts and if it's too much I'll be putting the parts on here for sale.
Anyone needing pic by pic step through to remove the roof lining let me know.