Hi Dom,
The changes should always go into the theme you are using as they override the parent CSS.
Hopefully UKFilmNet theme has a settings page that allows you to add CSS to it? If they settings page exists it should be found in the Administration side block under > Site Administration > Appearance > Themes > UKFilmNet - which is a link to that settings page.
As for the editor.css, well this has been a bone of contention in Bootstrapbase for quite some time, but as far as I am aware, the editor.css is in BootstrapBase and is (or should be) read by TinyMCE. Although it does not pick up the styles like ATTO does, and yet TinyMCE has the added ability to customise which I believe your previous helper who is, I believe in the EU working on a new project and cannot help you now.
I know I have helped you in the past but I was a bit of a novice back then, and still am in some respects, especially when it comes to managing big websites. But I can fix bugs if they need fixing, or restyle a page if it needs restyling. However your problems seem to be more self inflicted...if I dare to say so.
I'm not sure why the icons should have been made smaller, unless they are picking up so other CSS meant for some other elements?
The problem with the WYSIWYG not being honoured when you view it after saving, is perhaps because the styles in your TinyMCE editor are not in your theme's CSS file?
Actually, reading between the lines in all of your posts here of late, all point to CSS issue more than anything.
So perhaps a review of your theme's stylesheet may be a good starting place.
Does this all make sense?
Cheers
Mary