The stones mentioned at the sites above, Gordon, are not that big; 3-4 foot long (originally high?) by a couple of foot wide as a general rule (please see http://www.themodernantiquarian.com/site/2065/ingatestone.html where the largest stones there are now used as buffers on the corners of Fryerning Lane. These stones could have quite easily been bunged into a wagon and moved to Essex from further afield. I'm not saying they were moved but that, even today, there is a tendency for us to pick up a stone, pebble, shell etc from one place and take it home (in Hawaii there's still a tradition that says anyone who removes even a small stone from any of the islands will be cursed).

Just another idea to play with - ie the idea that the removal of a sacred piece of stone/earth/land from one place to another (from Wiltshire to your own land [circle] at Alphamstone in Essex for example) or the transportation of a piece of your own (sacred) land in the form of a stone to Silbury for example, formed part of an act of pilgrimage - we're in the Neolithic after all, there's not a lot that's going to last other than stone.