pwhole - that all sounds very interesting. Cannot alter the text of the book now tho'. I looked through several year books of Op Mole records in the DRO - plenty about exploring Bullacetree Mine eg D5289/5. Doug published printed copies at least of the following The Summary For The Year 1957 which I have.
In about 1960, there are two "caves" recorded just outside Harveydale Quarry. 40 years after that, I visited the site of the one on the south side - just a low outcrop of limestone but there may have been a bedding plane entrance there once. Perhaps Op Mole explored that.