Regarding the new tree planted, I understand the position of the National Trust, event though I am sure the "guerrilla planters" meant well. If the stump sprouts a new scion, does that tree get to remain, and the new one cut down? Or vice-versa? Does planting the tree potentially disturb buried historic artifacts that are near the wall? Also, does it open the door to people planting anything they want at other historic sites or landmarks?
I'm reminded of a recent case in Texas where vandals scratched graffiti into a stone cliff wall containing petroglyphs thousand of years old (site on public land). Professionals painstakingly did what they could to preserve what remained, far better than to have well-meaning amateurs try to remove the damage.