Hi,
OK so I’ve set up a subdomain and copied everything over, and…frustratingly, everything now works! Aaarrgh!
The site is exactly the same except for one thing. As I am using a sub-domain, my PRO license is not valid and so I could not automatically update it. So I had to delete and install manually. That’s the only difference. All plugins are still running.
Clearly a the original site is operating in a parallel dimension where the laws of physics are not the same!
Thanks
S.
EDIT: This may be a red-herring but, if I inspect both the working page and the broken page I see an error in the broken one. See secure note.