I think I have ressolved this now after searching Error 404 (sorry should have done that before!) Advice from The Dude:
“If you find you’re getting 404 Page Not Found errors after completing my modification code, you need to flush your rewrite rules.
To do this, you need to load (but not change or save) you Permalinks settings page (WP Admin -> Settings -> Permalinks).
I did this and it worked like magic, thank you : )
Just found thi spost which really helped – I agree that the small plus icon needs explaining better as it is a really great feature