Having resolved the central locking nightmare I psyched myself up to do a bit more.
I pulled down the ply bulkhead and removed the ply flooring and held my breath. I was pleased to see that apart from a few screw holes in the floor it didn't look as bad as I'd expected, not bad at all actually. The near-side rear wheel arch has had some welding and and I'll keep an eye on it and make sure it's treated before the proper floor goes down. I gave it a good sweep, vac and scrubbing and did the same to the cabin matting. It came up quite well so I laid the floor board back down and put acheapo reversing camera and I installed that, with a switch on the dash behind the steering wheel to activate it.
Next, a full service and and then underseal treatment.