I don't recall seeing anything on Nantymwyn in the SWCC library, other than this newsletter article and the Simon Hughes British Mining article, which, incidentally is reproduced on the Rhandirmwyn web site, along with surveys which I think are available on Aditnow. Mel Davies and Bruce Foster are sadly long dead but John Parker lives in the Pontypool area and might have info. Not sure how you'd get hold of him but might be worth contacting the Cwmbran Caving Club, which was also one of Mel Davies' clubs and to which John Parker was a key member.