From the Ramblers.
"Can I walk wherever I want on open countryside without trespassing?"
"No, even with the new freedom to roam in open countryside there are restrictions. First of all to qualify the land must be uncultivated mountain, moor, heath, down and common land, which has been officially mapped. This land is shown in yellow on Ordnance Survey Explorer maps. If you stray off this into domestic spaces, farmland or gardens you are trespassing. Land managers also have the right to close off areas of land for 28 days."