Hmnn, thanks for answering this question, I've wondered about it ever since! Back in '81, the dam was there but drain pipes at the foot were allowing the water to escape. There were concrete flags on top of the dam wall, bridging the gap between the dam and the portal. These could be moved aside to allow one to drop in and duck under the apparently walled-up portal - it was just the top bit of the arch that was walled. Water was probably welly depth, but I only had my walking boots and it was a misty, wet, sleety March day - not the best for walking around the moor with wet feet!
After Cornwall, the Grassington mines were probably the first mining field that captured my imagination. I used to make my own plans using available data from lots of sources, plotting water courses, rope runs, water wheels and shafts. It was apparent to me even then, that there were errors in some of the data.