Thanks for replies, I checked in on boatdesign.net as per gareth`s advice, read some posts by Tom Speer and others on the best software for hull design, and based on this I`ve decided to buy foils, rather than build them !! .
Man, those guys take the subject to a level I don`t even WANT to try & understand !
There`s so much you need to take into consideration that I never even knew existed.
Wouter`s concept of build it & test it might very well be the best option, except that after you`ve built 3 sets of foils that don`t perform well, you could have bought one set that does. Think I`ll try it, anyway.

Cheers
Steve