The Avebury ditch (specifically its depth) really does suggest a moat - it seems to be the only thing that makes any sense. The following may be of interest (even Burl couldn't help quoting it, although he doesn't agree with it).

"...by April fresh streams met at the Sambourne confluence, and new water sparkled in the moat. The monument at Avebury, or Aubury as it was known in the seventeenth century, was built to create New Water, and in the oldest European language, the Neolithic Basque tongue, Ureberri means New Water..."*

Avebury and the nearby Kennet (a word also of great antiquity and interest), the Swallowhead, Silbury and the Winterbourne all have one thing in common - water. But not just water, water that was born anew at certain times of the year.

* The Avebury Cycle by Michael Dames. ISBN 0-500-27886-5. Page 134.