Silbury Hill 180
Image by thesweetcheat

Other nearby mounds don't seem to have a solar connection.
Merlins Mound in Marlborough is said to be the tomb of Merlin and the Hatfield Barrow inside Marden henge looked like a Hat in a Field (Only joking, it was the HeathField)
None of these mounds were usefull for solar observing.

"(Only joking, it was the HeathField)"

Or the "Heathen Field" ....

Not sure what you're getting at here, I'm afraid. But I can say that I wasn't trying to claim that all mounds were used for solar observations. : )