Thanks all for suggestions. I only had an hour or two of wifi access last night and that was not enough time to arrange anything with Cave Access Ltd or similar. Also, as we were heading back South today we were not keen to go further north than where we were already (Croesor). In the event the weather relented for a few hours and we did a circular walk up Cnicht, round to Rhosydd, over to Croesor and back down the track to our starting point, before the rain came in again.
However, we will return at some point and explore further. Agree, masses of Cwmorthin and C-R we did not see, and masses of what we did see we didn't understand and were not able to put into context. Will try harder next time!