I suppose pronunciation before tape recorders is always speculative but I was interested that Samuel Pepys wrote about his visit to Avebury where he saw "Selbury" Hill in which was buried "King Seall".
All fascinating stuff, and before retiring to the Fox and Raven for my Saturday afternoon tipple :-) it might be worth bearing in mind that Z was often substituted for S in Wiltshire-ese - Silbury becoming Zilbury and cider becoming zider etc; that could account for the spelling of Silbury as Selbury as it's actually quite difficult not to pronounce Selbury or King Seall (when spelt that way) as Zelbury and King Zeal - try it and you'll zee what I mean ;-) 