Hi Nicolas,
Thanks for your pointers here. I relaised after considering your messages, that there was an inconsistency in the URL's being used to build the Colours category. You clicked on the colours menu item and it took you to a page with alias "glazes", but you clicked on a sub menu item of colours and it took you "/colours/submenu" instead of "/glazes/submenu". Clearly it was getting the wrong alias from somewhere. In the end, just resetting the alias of the Colours menu to "glazes" solved it and I was then also able to change it back to "colours" without it breaking. Don't know why it was getting confused regarding which alias to use - I still believe that I do not have any other menu items with either of these aliai. But it's solved, so thank you!
Best wishes,
Jonathan