OK some progress maybe; having removed the statcounter earlier, the Google audit report specifically for your mobile variant https://www.historicedinburghtours.co.uk/ looks a lot better; the errors have gone too! The safari mobile images may still be a problem, but I would fix the issues one-by-one.

Click image for larger version.  Name: HeT.jpg  Views: 78  Size: 18.2 KB  ID: 126879

Reference the IONOS SSL/CDN issue (unrelated to your images issue, are you using a separate Cloudflare account? If true, try this modification to the .htaccess file; it creates a permanent 301 redirect which should force http to https. We'll see

Code:
RewriteEngine On
RewriteCond %{SERVER_PORT} !=443
RewriteRule ^(.*)$ https://historicedinburghtours.co.uk/$1 [R=301,L]


#RewriteEngine On
#RewriteCond %{SERVER_PORT} 80
#RewriteRule ^(.*)$ https://historicedinburghtours.co.uk/$1 [R,L]


#RewriteEngine On
#RewriteCond %{SERVER_PORT} 80
#RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R,L]