After speaking with our surveyor, I think we're going to go 3mm steel instead. This will add weight to the boat, but it's a.) relatively easy to install and b.) going to provide a solid deck / hull join. Our surveyor recommended it and having done the figures / spoken to a local welder, we should be able to get it fitted relatively easily then weld all the deck fittings back on.
Thanks for your thoughts on this - next on the list is whether to or not to foam the steel hull.....