Huge thanks for such a detailed and thoughtful reply. I doubt the original video and audio are available, though I will check. These seminars were run in 2000 and are a highly specialised interest, selling to interested professionals within the field. The content is so good this tiny "audience" has been extremely forgiving to date.
The seminars are a combination of lecture and demonstration of technique split roughly 50-50, however the audio is definitely the more critical element. Having watched these on my iPhone I can attest that the video quality suffering a bit want seriously impact on appreciating the content. The audio is the problem. If I do nothing more adventurous than simply silence the sections of massive feedback it will be an enormous improvement, but I think with some careful compression to even out the levels it should be possible to make it an easier listen without my spending a colossal amount of time on it.
I'll check out the izotope plugins - I believe I own Bias Soundsoap (?) but haven't used it for a long time, so that might help too.
cheers and thanks again