I don't think targeting C4 will acheive anything. Targeting the NT is a possibility - after all, they are the ones supposed to be looking after the site 'for the nation'.

There has been another Eye created from Rocks on the shores on Loch Ness, and a crop circle has been produced somewhere as well. Lots of rumours of a similar stunt at Stonehenge, but nothing 'concrete' yet (sorry!!)