>Looking at Norths' work recently I am thinking now in terms of astronomical observation from the bottom of the ditch.<

Yeah, maybe. Guess you can go either up or down (structurally) to maximise observations.

Though much later than Avebury, the Cheomseongdae tower in Kyongju, South Korea is still one of the oldest observatories in the world. Built around AD 647 of granite, the twelve stones round its base, and the 30 layers of stones, are thought to symbolise the months of the year and the days of the month. 366 stones were used in its construction - one for each day of the year. There were no lenses of course but the prescriptive views from inside the tower would have aided observation of the night sky.*

The Avebury ditch may be serving a similar function though it <i>is</i> extraordinarily deep. Perhaps the reason for this is, as Nigel says below, that, "...there was water and silt at one stage, and later they dug the whole thing out - perhaps in an unsuccessful attempt to get down to a dropped water table? and thereby removed all evidence of silt."

* Photo and more info at http://www.orientalarchitecture.com/kyongju/cheomseongdaeindex.htm