"If a council found a site for a huge new housing estate to meet their housing targets and the only thing getting in the way was an "obscure" round barrow, for example, with a low theoretical monetary value, you can see which it would go."

Yes, that's exactly what happened with the Ribbon. Executive House plot values at that time were running at £200,000 so....

I guess there are two elements in the dynamic - how valuable is the asset and how much loot can be gained. Thornborough is valuable but the gravel's worth multi-millions. At least that's logical, in a twisted way, Tarmac were after the dosh. But Avebury's hard to take - the councillors' incentive was "tidying it up." You really can't anticipate or legislate against that, other than taking the decisions out of their hands. :(