I'd agree with only having two or three presenters (plus a special guest, maybe), and even then, their voices need to be easily distinguished. Any more than that and it gets harder to track who's talking.

Would it be worth trying a couple of different formats out? Maybe a totally free-form chat (though that could easily sound awkward), and then an agenda-led version?

With regard to special guests, I think that's a great idea and could really gain wider interest/publicity for BH.

Unfortunately, Ludovic Kennedy's informed his agent that he's no longer available. But perhaps Bill Gates and Linus Torvalds could be persuaded to perform "Who's on first?"