No, it seems that that pair of cairns are four thousand years old.

The peat is continually laid down and it traps objects. There is a distinct boundary in the peat that is shared by the base stones of the cairns and a seam of bits of broken wood. The trees were being cleared to graze sheep at the same time that those cairns were built. (A period of marked agricultural intensification !) Some of these bits of wood have toolmarks - there was one in my briefcase that day - but I can't tell if the blade that hacked the wood was made of stone, copper, bronze or iron. I have a section of wood out of this seam that is two inches across - it has distinct growth rings (*which can be matched against known growth rates by specialists*). I think it's a branch of Dendrochronology !

The profile of David's Cairns is not far from Silbury Hill - I've just found some more images to scan !