The largest mass trespass in a generation will take place in Devon today. Hundreds of protesters belonging to the pressure-group Right to Roam will descend on Vixen Tor, a slightly sinister-looking granite outcrop on Dartmoor a few miles from Tavistock. Since 2003, access has been banned. But given that much of Dartmoor is already open to the public, why the stress on this fairly small part, which is not?

Those marching on Vixen Tor say the reason is simple: a small part of the high moor where hikers have the right to walk at will is inaccessible because it can only...