Thank you to both Hugo and Robbie G. I haven't heard of the Nature Reserve and will try to explore that area again when time permits. We used to park by the old MOD buildings in Rhydymwyn, walk down the lane, through the caravan site, then pass through an old quarry before we came to the Leete path. There used to be a booklet on the walk which was produced by the council, I have a copy and that is how we found it all. I think the booklet dates to the 1970s. It gave a description of the history and the old industries that existed there. I recall it said many of the lead miners moved from Cornwall as they had experience of that work. There was the old mine manager's house which was derelict, he came from Cornwall too. I think that was close to Pen y Fron. There was a good circular walk from Rhydymwyn round Pantymwyn too.
I did read that the royal family had a top secret nuclear bunker at Rhydymwyn. That they were to travel there for safety in the event of nuclear war. There was one story that they had some storage for valuables belonging to the royals as well.