If you went down where I'm thinking there are about six adits along the hillside, although you can only really enter the ones at each end.
The quarry to the north did have entrances but that's the one that's been filled in since the Google image was taken, it looks like the East Quarry across the road now.
The Cortina was north of the north-most adit. You could potentially work your way from there all the way to the other end of the quarry, though water, collapses and air quality might scupper you. I don't know how far west the workings extend but it was clearly a large scale operation so they could go quite far- though again maybe not explorable.
I'm glad you like my pictures, I've been meaning to go back and get some better ones. I'll be interested to hear how you get on tomorrow.