The above will confirm whether a piece of land is registered; your question asks about unregistered land. The only way is to go looking for it! It may sound silly, but there are many little corners in the country that have just escaped being noticed. You then have to fence it, and maintain it (nothing too fancy required - just trim the weeds every now and again). This is the long wait time because you have to 'maintain' it for 12 years (for unregistered land) before you can make a claim for Adverse Possession. I am aware of a piece of land of about 0.25 acres in Berkshire where I saw this happen in 2000. Its in a semi-rural area, road fronted and next to another dwelling. Whether its the owner of the adjoining house that has done it I don't know, but in 2006 its half-way to being someone elses.