I’ve changed the profile so I’ll try it next time I find a youtube video that I like, which probably won’t be long, in fact I can think of one now that I might go grab the code for.

OK, next step would be to go to the ‘Users’ section in your dashboard, then to ‘Your Profile’. Untick ‘Use the visual editor when writing’. Click ‘Update Profile’. Goto ‘Manage’, edit this post, try again.
You’ll lose the WYSIWYG editor, but most of the functions are still there, and it can also help to solve issues that arise when you copy’n'paste into the WYSIWYG version.

ok, when you get a moment, goto the dashboard, goto users, edit your profile, and untick ‘Use the visual editor when writing’, click update profile, and try editing the post again. There shouldn’t really be any difference between that and the pop up box…. Can’t stand that editor anyway, never seems to do what I want it to. Looking directly at the code is ugly, but at least it gives you an idea of what is actually going on behind the scenes.
If none of those work, what plugins do you have activated right now? Maybe one of those is interfering?