did Stonehenge once sit on the top of Silbury Hill?

You've stolen my fantasy.
Maybe not the whole of stonehenge, that's a step too far (and would look gross, nouveau riche) but the bluestone circle yes.

All we have to go on is what it looks like, and that's a very unsatisfying construction, with something missing. It's an empty plinth, definite.

So what was on there? Not a single monolith, that wouldn't look right or in proportion unless it was fifty feet high. But a circle of stones, about bluestone size, would look perfect, the whole thing would be in proportion.