Wouldn't mind getting hold of that song. (HINT!)
When I was a kid - back in the 20s and 30s - my dad (who was a coal-miner) would often take us for a walk (make that a tough climb) over the mountains from my home in Aberdare to visit a friend in Maerdy in the Rhondda valley, so I can confirm that the correct spelling is indeed (to goodness!) MAERDY, not Mardy. Though we always pronounced it as Mardy, so that's probably how it became misspelled.