Stuart
The BGS records for the Pen-fford-goch borehole show a 3.47 m high cavity at c. 120 m depth, which is at a level at or just below the base of the Gilwern Oolite at c. 360 m OD. Another sand-filled cavity 0.47 m high occurs a few metres lower at 122 m depth. This is at about the same elevation as the Elliptic-Nunnery-Squirrel Rifts series of passages. If memory serves me correct (?), the location of the borehole is probably just south of the squirrel rifts area (its on the survey I think, which I don't have to hand), so it is likely that the BH hit a passage associated with this maze network. The borehole is also written up in the Bulletin of the Geological Survey of Great Britain No. 67 1978 (Barclay and Jones) which is available online on the BGS website.
http://pubs.bgs.ac.uk/publications.html?pubID=B04524. I recall some talk (heresay?) of a gamma logger being lost in this borehole, so it may be in the cave!
As for Big County - it's not totally clear how this bit of passage relates to the rest of the cave; I very much doubt the main conduit will continue south, but it probably fed into the Score-Gilwern Conduit at around 310 m OD link, a drop of around 30 m. Alternatively it may have joined the Indana Highway conduit at c. 340 m. THowever, the water in this bit of passage has since been captured down dip, like the Agent Blorenge streamway from Haggis Basher, and is probably a similar steeply descending vadose streamway - the 'Mystery streamway'. This will join the Beyond a choke stream somewhere downstream of the Riflemans choke, together with the stream in MS&D which is captured south in the Yellow Van area. The main MSD passage is a down-dip vadose passage graded to the Score-Gilwern Conduit at around 310 m OD.
The present Beyond a choke streamway will continue south beyond Riflemans, and may have led to a former resurgence at an elevation of c. 270 m in the Blaenavon area. The stream has since been captured into a lower level streamway which then flows beneath the Lwyd to the springs further downvalley.
Still huge potential in Draenen for lots more passage!