I've seen a lot of tree roots and swirly fossils but this is very different. I'll go back at night with a torch sometime and take some more pics.
Here's one of the mounds I found
http://aveburytour.mysite.wanadoo-members.co.uk/Untouchedmound.jpg
It has no signs of excavation, rabbit /badger holes and is in very good condition.
There is a single sarcen laid flat on the top of it.
PeteG