Had a good look around Cefn yr Ogof and Gwrych yesterday. My initial doubt about a passage from the Dulas Valley to Gwrych is now not so incredulous.
The veins crossing the hill west to east appear to have been extensively worked from the surface. Although the OS map shows 'shafts' I didn't come across any of them. The GOES link above does have them descending a shaft, but does not describe where. Having crossed the hill to Gwrych, I came across the adit quite easily. It's not particularly spacious, and terminates at a gradually rising floor level blockage rather than a dead end. Some of the references I've seen state it doesn't reach the vein, but given the closeness of the vein, I'd be surprised if it was abandoned before reaching it.
Having had a quick look at the Cefn yr Ogof caves, I had a look at Ogof Dulas. This is an old phreatic passage, about 200m long that has been utilised by miners. This must go somewhere as the draught and temperature drop is pretty impressive. A hole in the floor was passed, which leads to a series of stopes almost reaching surface - to which some of the airflow can be attributed. I followed it for most of it's length until the prudence of carrying on with just a hand held torch and little else brought me to my senses. The end, apparently is a muddy boulder blockage. But given the airflow, this must continue, either as an adit, or to some shaft linking to the surface.
There are signs of visitor activity, but I doubt if anyone has had a proper go at extension since the 70's. I think that there might be some stuff to find here.
Robin