>...they then admit that the amount of collateral damage that will happen in terms of collapses adjacent to the new tunnel actually CANNOT be known!<
I'm a little confused here Nigel. Where is this new tunnel? I don't trust EH any more than you do but the policy they seem to be advocating (re: http://www.english-heritage.org.uk/server/show/nav.8820) suggests a controlled back-filling (with chalk) of all existing vertical and horizontal tunnels (or am I missing something here?).