Hi Steve
I am just across the Bay in Stainton with Adgarley, I am not sure about posting links on the site but try this (https://sites.google.com/site/escalierzazou/) if it is not allowed PM me and i will send it to you, this project can only be made on a CNC machine and it also is shown with some steel sections to the same profile for stiffening i think if all the sections were glued up it would have more than enough strength, as far as i am aware these don't appear to be available in the UK so a good opportunity to get into designer stairs, hope you find it informative.
Kindest Regards
Mike