I have test your site. It seems some of your third party plugin causing this issue. Because on you edit page I have seen the warning: “Failed to load plugin url: https://www.invernesscabling.co.uk/wp-content/plugins/wp-fsqm-pro/static/admin/js/ipt-fsqm-tinymce-plugin.min.js”
Kindly deactivate all of the third party plugins and clear your site cache, then check back. This will surely fix the issue.
In case if the issue is still there then remain the plugins as disabled and let me know so I can test with theme default plugins.