So, I made a certain amount of progress this weekend.
Windows and ventilation louvres are fitted, it looks a little rough at the moment, but final sealing and then paint will improve the looks (and yes, I forgot to take off the protective film before I fitted the side window).
Also had an exploratory dig at the floor, apart from some planks being made of cheese, the metal structure underneath seems solid. Hopefully I should get the floor out next weekend.
The only iffy bit is one bottom rear corner where the aluminium skin has reacted with the steel angle, it's only a small hole, so should be a simple repair.