After asking many, many people (and the most likely helpers being no longer with us), a bit of an update and a plea for more info. After chatting with Dave Penney a while ago, it appears that he was digging out one of the backfilled shafts in the floor of Middle Bank Gully, below the shakehole, and that it went about 3m to bedrock, so he had pretty much given up. PB Smith was digging in the shakehole itself, but I'm unclear whether this was simultaneously done with Dave's dig. Whatever, it appears that the Duchy got wind of it, and that given that the shakehole dig wasn't exactly safe nor sound, told them (together or separately I'm not sure) in no uncertain terms to pack it in. If anyone knows more, or can clarify (or contradict) what's already noted above, please pipe up.

The reference in COPD appeared in the early 80s I gather, and has remained unchanged ever since. It would be nice to be able to update it one day, especially given where it is...