Well, as we are out of lockdown in Wales, I had a trundle today. Parked by the Pont Newydd bridge, and followed the leat across to the Arddu. The 'mine', corresponding to Davel's 1900 map is marked on the ground by several grey, flakey pyritey, with red outer rimmed spoilheaps next to a river. At the base, there is a substantial flow of water, but I suspect that's the river seeping into the spoil rather than an adit.
The actual working seems to be a 'quarry' higher up between the track and the pipeline. I didn't climb down, but I didn't see anything which would be underground.