Not sure if this has been mentioned before, but saw it advertised on the wall in our local archives last night.
http://www.bgs.ac.uk/nocomico/ The Mining Plans Portal is a partnership of public sector bodies who have come together to make information about the extent of historic underground mining of non-coal minerals in Britain more accessible.
The aim of this website is to make information about underground mining records available directly, as well as to guide you to organisations that will be able to provide more detailed information when you need it.