not on deeds

When my parents bought their home in 1971 there was nothing stated on the deeds  that was a mine outlet in the back garden .Forward   40yrs  Coal board turn up to check it ,he  wasn't sure exactly  were it was pointed to a area  just asked if we had notice any movement in the ground.
I have now been told that the house is only worth half the value because of the shaft. 
Just wondering if the Coal board should fill the shaft 

Beryl - it's legal advice you need here with regards what rights and more you/the Coal Board have re the mine outlet and impact on your land/property