OK - review based on your posts
These custom styles you are using in TinyMCE - how have they been created? What are they actually applying? Where are they picking up the styles/css from that is not then being matched in your theme css?
Have gone back into your site this evening Dom. It appears that someone has used a custom configuration on your TinyMCE - hence the reason some of what you were expecting to see disappeared when you switched to Atto.
{
"theme_advanced_blockformats" : "p,div,h2,h3,h4,h5,h6,blockquote,dt,dd,code,samp",
"content_css" : ["/learning/theme/styles_debug.php?theme=ukfilmnet&type=parent&subtype=bootstrapbase&sheet=moodle","/learning/theme/styles_debug.php?theme=ukfilmnet&type=parent&subtype=clean&sheet=custom","/learning/theme/styles_debug.php?sheet=ukfilmnet&theme=ukfilmnet&type=theme","/learning/theme/ukfilmnet/style/tinymce.css"],
"style_formats" : [
{"title" : "Grey background", "block" : "div", "classes" : "grey_background"},
{"title" : "Stripe background", "inline" : "span" , "classes" : "stripe_background"},
{"title" : "Caption", "inline" : "span" , "classes" : "caption_style"}
]
}
To be honest that's outside my area of expertise but...
What it looks like to me is that someone has avoided the use of the editor.css by applying custom configuration to TinyMCE, so we can probably ignore that.
BUT in doing so, they have created some drop down styles for you that do not seem to be reflected in your main theme styles and also that custom configuration is looking for a style file ukfilmnet/style/tinymce.css which does not exist in your theme - hence the 404 error.
I hope thats a small step forward, and something that others in the community and pick up - I am fully aware that it is not the kind of step by step guide you need, but maybe someone can use the information to move closer to that for you as I wont be able to look at it in detail again until at least the weekend.