I've also walked along a lot of the river when dry and can't recall Silbury being hidden.
I therefore feel extremely doubtful about the antiquity of the Silbaby mound. It isn't a small mound; it is big and very, very obvious indeed.
Furthermore, whilst nowadays the road here has a steep cutting (and no obvious tracks) to the north, in times past this was not the case.
No, what I think happened was that some time in the sixties or early seventies, the main road was widened and cut much more deeply into the hillside to accommodate this widening; indeed this is the only way that the work could have easily been accomplished. Cutting a road several metres deeper into a hillside produces much spoil, which has to go somewhere. That somewhere was the down-slope side of the hill, and the pair of houses I mentioned were probably purchased to provide this dumping ground.
I therefore think that a century of land surveyors were not blind idiots, but extremely skilled men and women; they didn't see Silbaby because it wasn't there then. The layered structure seen by cutting into the mound is simply an artifact of the sequence of dumping of the road cutting materials; the workmen wouldn't care about how the material got dumped in the least; they'd just want it out of the way.
What do you think?