As a non-believer par excellence I would have to disagree totally with the half-hearted and amateur non-believers above who claim there no such thing as sacred, but then I guess my definition differs.
In any case, our notions of sacredness aren't central to what I was wondering about, which concerned the motivations and attitudes of the original builders. Were they building a sacred (i.e. religious) landscape? It seems often to be taken as obvious fact in much that I've read. Or are we putting our own construction on a much more complex bundle of different motivations that happen to now be observable in one particular location? I guess there are too many anomalies (and I really enjoyed your stuff Brigantes) for it to be other than the complicated answer.
I would add two further complications to the bundle:
First, as I've rambled elsewhere, I feel and therefore reckon they felt, that sacredness or significance attaches to lots of points in the landscape, not just to monuments. When we look at the landscape today there's nothing to prove that to us, yet if it's true we must be missing much or even most of the point.
Secondly, invaders. Once you get those the slate is wiped clean and that which was previously sacred is open to desecration or loss of significance. A speculation: is Silbury 3 a high-tech cruel edifice deliberately built upon the deity of a conquered people, Silbury 1, burying it forever and humiliating them? Perhaps they were enslaved and forced to do it themselves. Scary, but an utterly human thing to do! That's just a bit of raving, with only a bit to support it, but there's no strong evidence it's wrong. If it was right then what price Avebury's sacred landscape?