Problem is it is a tin mine over 200 years old, the company which owned it long disappeared and any documentation with them. Its quite common now to expect a mining survey with a planning approval application, if the general area was known to contain historic mines. But with a mine closed over 200 years ago, and a patio built maybe 50 years ago when regulations were less rigorous, who knows? The patio may well not have needed planning permission. There are companies these days who do a lot of business finding and capping these mines.