I remember being puzzled by that location when the video first came out. I knew it was in the Ffestiniog area but not sure quite where, it was not a quarry I recognised. At the time I had been round pretty much all of the Blaenau quarries, including Manod and I am sure it wasn't amongst them. However, there was one quarry I had never been to, Graig Ddu. Sure enough when I went there a year or so later I was able to identify the adit, and the incline head (you are correct Vanoord it was a drum house ruin). This was before the major works subsequntly done by McAlpines who now work it as part of Manod (CwtyBugail). I doubt much survives as lot of the area has been tipped on since.