The level is open and a trip can be taken finishing under Glog mines at Skinners shaft. You pass through the logaulas workings on the way through. The level is well over a mile long. The entrance is at the Count house in Pontrhydygroes, which is owned by Peter Harvey (of Llwyernog Mine Museum). He has always allowed access to those interested, contact him to confirm. (It was an trip for NAMHO a few years ago).
Incidentally I've always thought of this as 'the' Lefel fawr.