Can’t remember who she was now but some archeo, writing in a book years ago, said Silbury couldn’t be seen from within the Henge. Silbury can be seen from the Obelisk but whether that’s always been the case is another matter. Silbury can also be seen from the Bank in the north-east quadrant. Dunno if there are any other places in, or on, the Henge from where it can be seen though (unless you climb one of the stones that is ;-)