Thanks moss - the JR site looks interesting.
Just compared Camden's illustration with a diagram on page 137 of Mike Pitts' <b>Hengeworld</b>. Camden seems to show ten lintels in place while Pitts shows nine, with three more on the ground. Looks like the lintel that was still in place in Camden's time is the one that now lies between the two standing stones in the north of the circle.
All this must have been written about before though :-)