Hello guys, good news!
My hosting tech and I went through some series of diagnosis.
He narrowed it down to the permissions in the theme folder. There were the meant to be 0755 & 0644 permissions on theme files.
He ran a script to correct the permissions and all works now.
Have a quick look at the permissions in the theme file you sent me to confirm this is the case at your end.
Hopefully it will be the solution for everyone that might have the same issue as I did.
Looking forward to hearing from you with a confirmation of our hypothesis so minds can be at rest ๐