I was looking at these things at the Southampton boat show a couple of years back. I thought they seemed a bit cumbersome however I read a review in a boat mag shortly afterwards that said the on water performance was really impressive (they also hold the world speed record for amphibious craft!). Neat idea but a bit pricey. They are built in New Zealand, with hulls and consoles made from aluminium.
Jim having seen your very impressive trailer engineering at Penmarlam, I'm sure you could develop something similar for your Osprey