Mark, the fact the stones are heavy would hardly have prevented the church from pulling them down. If the builders of Stonehenge could pull the stones from Wales, it would hardly be a difficult thing to pull them down. They certainly had the manpower.
Your second point also appears weak to me. I doubt that many Englishmen, or womne for that matter were aware of the circles unless they either travelled by them or lived nearby.
These website suggests that some were destroyed by the church. The point about superstition may ultimately be the reason more were not destroyed.
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ancienthistory.about.com]
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istina.rin.ru]
Finally, I think it wise to keep an open mind as to what we know for sure about Blake and what has been assumed through analysis of his work.