Thanks for pointing that out Rhiannon.
The avenue there is certainly pointing south of Silbury. But is it the Sanctuary or not? Looking again at a different scale OS map, I can see a 'Site of Stone Circle' marked just a short distance south of the Sanctuary (which is marked separately) NGR SU 118 679. But on MAGIC, this is described as a Bowl Barrow.
The mystery deepens? Which is the 'Head of the Serpent'? Which is the double circle?