Resolved after 15 hours of looking.
The issue was a .svg file which I could not find and for good reason. It wasn’t on my website.
When this issues occurs due to an image, it’s fairly simple to find, but this was a .svg which in my case threw me a red herring.
It was in the Child theme (where I kind of knew it would be if it wasn’t in the Custom CSS), but it was hidden under ‘Fonts’.
The font was being dragged from a http://www.w3.org/ site, but was of course HTTP
I simply added the ‘s’ and angels started singing.
Just wanted to post this as it would seem no one else in the world has ever had this problem and it almost cost me my sanity.
Hopefully no one else will have to suffer this way again.
Child Theme > Fonts > http://www.w3.org………..