Check with the Land Registry, it really doesn’t cost a lot. If the land isn’t registered it might be worth getting copies of the titles for all the properties adjoining the land, to see if they have any kind of rights over the land. If the property at the back isn’t registered either then the land might well be theirs.
If the land isn’t registered and you don’t have any clues as to who owns it, it can be very hard (and expensive) to figure out who owns the land, and they may not exist any more. As above if it is just going to be a liability then don’t worry about it too much, and don’t let it put you off. It is likely you’ll be able to claim it in the future if you want to.