You seem to have covered most aspects Nicki, there is also this charter which is basic common sense
http://www.stone-circles.org.uk/stone/charter.htm
I have noticed that English Heritage are also putting something on their information boards about prohibiting unauthorised metal detecting at sites they are responsible for. Its easy to forget about issues such as erosion which is a big problem at Avebury and on West Kennet Long Barrow as people always walk on the banks and to the end of the long barrow.