I just found Penny Drayton's article - very good. It seems that the word "toot" is related to "teat" and means a nipple! Apparently toot hills had nipple-like structures on top of them - presumably shelters for the watchers. So that is what the flat bit on Silbury is - a platform for a nipple-like building! The whole thing is a big tit and must be in honour of the great Earth Mother. I just know I'm right because my crystal skull tells me so. ;o)