Maybe we should all nominate a site or two for gardianship (with the owners permission of course) that we feel are at risk?

Well the NT has an Avebury Guardians scheme for litter-picking and such like, but they retain fairly tight control over it for obvious reasons.
I suppose you could write to EH asking if its OK for you to keep a couple of places tidy (with the owner's agreement) and see what they say.