There's lots of big glacial erratic sarsens as far down as Essex, including one allegedly appropriated for ceremonial use in the Bronze Age.
See picture here, a village near Stanstead...
http://tlhs.port5.com/our_artifacts.htm

and it says:
It has been agreed that the Sarsen Stone ( pictured above) will be sited as a monument at the Four Ashes crossroads adjacent to the Takeley village sign.