Dave: I know what an estuary is. What I would like to know is how does it apply to an accent. Is it a region in or near London? Does it refer to a particular river area? The reason I said a true cockney accent might be unintelligible to many Americans is because I was once preparing a production of Shaw's "Pygmalian" and we broguht in an authentic Cockney to teach some of the cast members and found that the accent was so thick that people in the audience had trouble understanding the actors. I've also read that when Stanley Holloway created the part of Doolittle in "My Fair Lady" he had to tone down his accent to get the words accross.
Americans in rural places who aren't used to hearing foreign accents often have trouble following them.