I would imagine that there's not much difference in law between landing a glider in a field and landing a powered aircraft, graduating up through microlights to light aircraft designed with that in mind.
Operational suitability is of course another issue altogether.
When did a glider pilot last call the Plod to get permission to land? They often, so I'm told, land without even asking the landowner first! That's wrong, of course; simple courtesy dictates that they should call first (with a loudhailer) and climb away if refused.