Thanks for the comments, guys.
Graham, you are right about the fixings, I do need to come up with a better method. Perhaps I could make the mating surface thicker, perhaps two lays, and conceal one of those spiky nuts between the firsat and second layer. I will have a think about it.
James, I personally don't care for pine and it can cause problems with resin building up on cutters etc. It probably wouldn't cost a lot more to use American ash instead of pine, and the value would be much higher, pine being quite common at the moment. After I've tested the market with this stuff I might well do one in hardwood if the painted birch ply is not popular enough
John