rossgiles,
Taking the PS first, try
http://www.harefieldmarina.co.uk/
Non BS 1088 ply usually has unseen voids within the layers and you must ensure waterproof adhesives were used. Three layers of 8mm and one of 6mm will produce a cold moulded hull of 30mm+. Extra bias tape inside and out at the turn of the chines for strength and abrasion resistance and one layer of 450gm cloth should be enough both inside and out. Sam Devlin's book will give you excellent advice on cold moulding with sheets of ply.
http://www.devlinboat.com/
The flat bottom of the hull could be built with planks laid athwartships and sandwiched between two layers of ply. The epoxy/glasscloth layer encapsulates the wood and keeps the water out. A hull that will be around 35mm thick will not need many frames (or ribs) as built in furniture such as bunks, shelves and lockers etc., will add rigidity. Nor will the hull be lightweight with fresh water, diesel fuel, waste water tanks and coal for the central heating boiler.
Pericles