Sheila Harper from Old Cornwall Society sent me the following info:
I'm researching local mining and quarrying and was trying to find out a date for when marble was mined from the Cathedral Cavern, Porth so looked at ukCaving and thought you might like this info -
It was the Banqueting Hall that got blown up in 1987 by English China Clays Ltd. NGR 8272 6310.
Re the Pisky Holes, Fistral Beach, Newquay - there are still some there under the cascading Hottentot Fig. There is one you can climb in - about the size of an oil drum.
Regards, Sheila