Nigel has also got it right when he says, “...why not honesty boxes?” That’s what’s in place at Stanton Drew. And I wouldn’t mind paying over the odds for a decent coffee, with a good view of Avebury, if I knew the money was going into the upkeep of the place ;-)
So why not honesty boxes at all sites? I'll tell you why...because a great deal of people are, although not completely dishonest, unlikely to chip in when there is nobody else about to observe them NOT putting anything in. They just don't work, hence the absence of them through that and theft.