I looked in the Heritage Gateway website to find out what records there were of the shafts (see, fanfic DOES teach useful skills!) and I found that the record for the nearest documented shaft, Taylor's shaft, was empty, so I emailled to find out what the record said. Not only did they send me a copy of what the record should have said, but they also sent me this lovely map. My mum's house is roughly on the red dot. Look at all the mining bits! Tragically, it didn't say how deep Taylor's Shaft was, although the Internet thinks it was 720 feet deep!
The records only go back to 1820ish, and this mine, Devon Friendship (doesn't that sound cuddly?) goes back to at least 1740, so there could still be something undocumented under her house. But we hope if there is, it's a small, undaunting spring or something rather than a Giant Mining Hole.