A problem we've also had to deal with in Derbyshire, with abandoned mining-related holes on NT Access Land. Trouble is, the NT don't know where most of them are either, making it very difficult for them to fulfil their responsibilities. Working on one site for a long time has meant we've been able to log quite a few, and we'll be giving them a list of grid refs so that they can be inspected. Cavers liaising with them will also hopefully preserve safe access, rather than just blocking. So 'accidental' discovery, by someone falling in, is obviously in everyone's interests to be avoided.
The responsibility issue is also very relevant through, as a person gaining access to an entrance they've discovered safely must also make the decision beforehand whether it's a worthwhile risk. Blaming the landowner isn't enough, as they could have just not entered, removing the risk entirely. Having seen some of the people accessing the land around Castleton, I'm always stunned to see how little preparation some folks have for dealing with a very high 45? slope of slippery grass and limestone beddings, with a public road or footpath at the bottom - not including the hidden holes they might be crossing. We recently found a dead sheep at the bottom of Cowlow, and the skid mark from the top of the hill to the bottom was obvious. When we got to the top, it was clear that it had just lost its footing on an outcrop, and down it went.