Try looking at the search using advanced Search
Leave the mine name blank
Set country to United Kingdom
Set the drop-down that appears when UK chosen as OS sheet SH
Set mineral as slate
Now click the button Show as Map (Google)
Zoom/move the map to Penrhyndeudraeth etc etc
One issue is that various sites have more than one name, so take a look on map view. I'm not a huge fan of Google maps outside of large towns, but it should work. Stay on this site/forum too, as quite a few of the members are from that area and will know far better than me.
Expert in incompetent tomfoolery