Just to clarify, 'twas Pete's rediscovery, and he then showed it to me and Jimit. (Best to have that clear, in case it turns out to be a rubbish dump!)
The correct procedure to go up Marlboro Mound is to get advance permission from the school. However, it's easily accessible so I suspect there are those who don't.
I have a suspicion that minus intervening trees it may be visible from the end of the Ridgeway within yards of the Sanctuary, or from a few feet above the Sanctuary.
Silbury, Silbaby and The Sanctuary are all in a perfect line.
Does this mean anything?