In the U.S. you can sell a home "as is" and disclose the tunnels as a safety risk. The buyer can fill in the tunnels if they don't have a need for them. Homes are sold all the time with defects, code violations, etc... Sometimes the land/location alone is the primary value of the property. That is the case in my neck of the woods. Several of my neighbors had dilapidated homes with rotted wood, 100+ year old redwood septic systems and still sold for seven figures. I have no idea if you can sell a property "as is" in the U.K. I also have no idea in what neighborhood Colin resides.