Thank you for your replies. The wing is (unfortunately) not warped, and the fin / vert stab, call it what you will, not receiving sufficient air flow is a valid suggestion. It does not however explain why the original fin / rudder assembly which is of indentical size, shape and area performed perfectly well.
You can imagine my frustration, at not being able to identify a cause.....
Thanks again for your replies, they are of course most appreciated
