I suppose the official and sensible answer is that you should always politely ask the landowner before descending on an overgrown site armed with machete and flymo. Because they might say 'yes' and be pleased you were volunteering. But what if they say 'no - and get off my land'? Which is why the place is so overgrown in the first place. Then you'd wish you'd just snipped first and asked later. I guess if the plants aren't damaging the site then there's no recourse to complain to EH or whoever. But what's the point in a site you can't see because it's under a heap of brambles? Is all property theft? etc
OBviously I'm speaking hypothetically here and have no burning desire to go and hack at any particular overgrown shrubs with one of those handy pocket saws.
What do people reckon.