After a morning's work, I've cleared out the carpets, rear seats and belts. It was also an insight into why the passenger seat wouldn't slide forward - there's a metal rod that should connect the runner catches on both sides which had come off. Fixing that let the seat slide forward giving me access to the rear bolts.
The rear seats after some headscratching revealed two small bolts hidden at the sides holding them in, and in an upwards lift they come out.
And now some smelly wet carpets are removed. If anyone wants the old seats, boot liner etc let me know quickly! The floor mats are virtually brand new so they are going on ebay.