Thanks Saint,
very useful comments. here's a pic of the finished floor construction. 75 x 50mm battens, 50mm Kingspan (450mm wide) two layers of 9mm WBP FSC hardwood ply, copious screws. It feels pretty solid once screwed tight.

Looks good - but watch out for your plywood quality. WBP doesn't exist as a recognised standard for plywood any more, so if that was sold as WBP, what actually is it????

Jim,
Thanks for your very helpful comments, I shall take them on board.
You mention the table-saw; what do you use this for? To rip the studs I presume, and not the Kingspan?
I have just bought a table saw and it has opened up a new world of possibilities (e.g., being able to rip, say a 12mm thick strip of timber for trim; that stuff always costs a fortune in the big sheds).
I am worried about weight, the chassis is only good for 900kg. I've decided to lighten my construction by using 50x50 , rather than 75 x50 for the wall studs, saving 100kg -200kg overall. ( and 50mm PIR rather than sheep's wool) Hence the table saw. I'll be ripping the 75 x 50 (or 80 x 57 as some bits are) down to 50 x 50, saving weight and hopefully getting a more uniform stud to work with.
Where is the "condensation air gap" ? Just under the tin, above the roof insulation?
I really want to see the roof purlins, they are lovely planed douglas fir. It makes the ceiling more complicated but I think it will be worth it ( I read somewhere that this is called an "expressed timber frame" and that's my ideal) The plan for the roof at the moment is 75 x 40 douglas fir rafters, running longitudinally, supported by a central arch (plywood laminated together), 3,5mm veneered ply bent on top of the rafters (purlins?) which will form a finished ceiling surface, vapour barrier, thin foil insulation, 50mm battens, nailed to rafters below, with wool insulation between, then the tin, corrugations at 90 degrees to rafters & battens.
Would the humps under the tin be sufficient for a "condensation air gap", I wonder? I was planning on using eaves filler though, to keep out beasties, which would prevent air flow. Hmmmm.
I know what you mean about the corners, it worries me a little too. In situ the corners will all be jacked anyway, using the original caravan jacks, but for moving the hut between sites, I don't want it sagging, creaking, tearing apart. I do have another caravan chassis which has the "wings" you mention already in place, but I was keeping it for Hut2. It gives me an excuse to get some welding gear which I've always wanted though!
I was initially quite critical of the flimsy construction of most caravans but i've now realised its a fine dance between strength and weight and those guys clearly knew what they were doing after all. Even if they do seem to use cardboard a lot. Humble pie moment.

I'm using vertical board and batten cladding, see mock up below, the douglas fir is a lovely pinky colour, although I guess it will fade.

Wul, sorry...been away for the weekend. Yes, I use a table saw to rip the kingspan to the exact width to fit between the studs as it makes for a very accurate cut using the fence if the table is wide enough!
The air gap should be after the insulation and before the cladding or roof sheet. If you want to leave the purlins exposed, i'd put your ply on top then vapour barrier as you say, then 50 mm kingspan (or whatever thickness you require), breather membrane, 25 x 50 batten to form the (25mm) air gap then roof sheet. I say this as an alternative to your foil and wool combo as it doesn't leave a 'defined' gap so the wool will most probably end up filling the corrugates of the tin reducing any airflow. The battens can be fixed by nailing/screwing through the insulation into the purlin below.
Might be naughty but I've never really bothered with eaves fillers as the standard corrugated profile only leaves an 18 mm gap, so only the small beasties can get in! .. or if you prefer to have eaves fillers you might still get enough ventilation at the ridge. Or use a mesh/ netting for filler so it stops the beasties but not the air!
Love the tones in the DF, I've been using a lot of Larch of late as its more available, but the Douglas is more pleasing to the eye...it's going to look right proper!! good work!
Jim

In reply to "Timber" re the WBP standard.
Yes I've found this confusing (new thread about this in "self-build")

I used this stuff, on the advice of the merchant. What do you reckon to its provenance?

Saint,
Yes, that sounds like a plan! I could use a circular saw for that.

I'm not sure what the difference between XPS and EPS is, I'm off to Google it. Thanks.

Posted By: SaintBe careful wth a circular saw, at the wrong speed the blade heats up (and there's no where for the heat to go...its inside an insulant!) the polystyrene then sticks to the blade.

My groundworkers had good luck with a petrol-driven Stihl saw/cutter (sorry, don't know what type of disc they used). It did get warm but made a pretty clean cut through high-density EPS. They tended to prefer a good quality hand saw for the regular low density stuff, which was surprisingly quick and effective.

A lower case "e" looks a bit like a balloon; a balloon will expand if you blow into it.

An "x" on the other hand is a perfect shape for extruding.

Does this help?

David

For me (rather dyslexic) I prefer that E comes first in the alphabet - the product came first, it absorbs more water than XPS, it comes in only 2 colours, it isn't as strong, it isn't as good an insulator and it makes a big mess when you cut it, X comes later in the alphabet and scores better on all these points though costs more :) - whatever works for you of course.

Oil rigs in then North Sea where abbreviated to a single letter then the country designator UK
Worked well until they were going to name the 6th one.