The way in which internal voids migrate upwards and appear at the surface some time later has been analysed in great detail. To cite "no current surface evidence" as a sign of "no new internal collapses" is just a joke.
Spot on Nigel - though given the fact that only a couple of weeks ago Pete was claiming that, "Stukeleys sketches don't show the step."* when Stukeley clearly does show the step ( http://www.avebury-web.co.uk/plates/24.jpg ) it's hardly surprising that he would post yet more inaccuracies along the lines of, "If there had been any more serious internal collapses they would be visible on the surface of the mound."
* http://www.themodernantiquarian.com/forum/?thread=44389&message=556302
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