Now for the inside

Hmmm. When in doubt stop for a cuppa and pretend to be thinking

With the outside looking reasonably shipshape it's time to go inside. First job is fitting the battening that the inside surface panels and boards will mount to. Even in the seminal narrowboat builders book (seminal because it's actually the only narrowboat builder's book) and called unsurprisingly The Narrowboat Builder's Book, battening is dismissed in a few paragraphs. Which isn't a great help when you're confronted by some random tags sticking out of the sides which the battens fix to and rather more empty spaces where you think there should be more.
Even the all-seeing internet isn't much more help. The dozen or so pictures of battened out interiors I found were all quite different. Every shellbuilder prepares differently and every boatfitter has his favourite way of doing it.
So after much musing, some swearing and general advice seeking I think I've finally worked out how to do it. Or I hope I have.