I listened to this tonight, it didn't have an interesting revelations but was nice to hear the round table chat and they even managed a snippet from Matt Smith.
Are there no surviving archive interviews with Hartnell?
Possibly not. He was on Desert Island Discs (the series that alternates with The Reunion in six week batches), but that doesn't survive, and I think it might have been before his time on Who anyway.
That's a real shame, it's one thing to hear his relatives talk about him but there's nothing like getting the actors personal memories direct.