Add and Customise Pages permission denied yet I am site admin have full control

Ok so I think I have solved this myself! 
It seems as though navigating to the page library and clicking 'New' and selecting any content type doesn't work - regardless of permissions.

However just going to the home page and selecting New / Page works perfectly and gives me loads of options and stores them happily in the Page library!

Is this just a functionality change on modern sites maybe?

Anyway, found a solution.