Julian suggests that it is named for Suil because of the "eye goddess" game it plays with wanderers in the Avebury ceremonial landscape. Suil, Julian argues (and I agree) is the name of an ancient Sun (and eye) goddess.

http://www.themodernantiquarian.com/user_profile.php?id=3401&show=weblog&weblog=11528

Aubrey called it Selsbury (it was in the selkley hundred) but locals always called it Zilbury.
Hence the legend of king Zil on his gold horse.