Anyway I acquired some offcuts of 4mm ply from my friendly DIY shop and after prototyping in cardboard as demonstrated by Crispy, I proceeded to the build.
Anyway this has progressed and the hull is now coming along nicely. Here are a few pic:
Sides and centre base installed along with many of the ribs

Bow and stern lower hull installed and prop tunnels started. Much help and advice here from Panther Ausf G without which this would have been a disaster build! - Many thanks

Prop tunnels installed - I cut these from an old silicone tube - although they are a bit flimsy and will need strengthening with epoxy/fibrglass before installing rudders and props


Stern reinforced so that epoxy and reinforcing frames will provide a firm structure for the rudders and props

Bow reinforced to be able to support ramp hinges

One lesson learned so far on the build was that the offcut 4mm ply I bought so cheaply has given many headaches as it delaminates at the drop of a hat and isn't terribly strong, so much reinforcing of joints required (probably not a bad thing!).