Thanks again - Pilou thanks for you further investigation bugging me why the outside edges grow in length - I have made an analogue version in a light gauge steel it seems that a jelly bean shape is where it whats to be without distorting or kinking the shape. I tried putting an extra twist into the 3d model to see if it relaxed it but no change. The metal prototype I have made is only the x1 180 flip in the surface.