Okay, made some progess with this...
Got the headlining brought over, so I've fitted that. It's not mint, but it's decent, and no drooping at the back which is good.
I've laid the carpets down again now everything's dry. Still not got the holes welded - I'm going to book the MOT at a garage that does welding too, with the idea that I can drive to the MOT centre for the MOT, and get them to weld the floorpan while it's there. Still, that's some way in the future yet...
Other option is to get a generator and MIG welder, and do it myself, but that may be costly.
I suspect the water came in through three places - the blower motor cover in the scuttle panel, the sunroof seal, and the door membrames. Surely if I get all of those fixed properly, the leaks will stop?!?