There's quite a few glacial erratics, bearing the name Thunder Stone, kicking around the Eastern margins of Cumbria, here's a couple more
http://www.themodernantiquarian.com/site/3154
http://www.themodernantiquarian.com/site/3135

Elsewhere in the country a thunder stone or thunder bolt is an old country name for a prehistoric stone axe or a flint arrowhead. They were believed to be good luck charms and were often stashed in the eves of houses to protect the home against lightning strikes.

cheers
fitz