I am not an authority on the subject, but from what I have read Llygad has been dye tested to caves in the Herbert's Quarry area, with speeds suggestive of open passage. The potential distance and vertical drop between the two points is greater than that of OFD. I think due to the above, a NE/SW direction is generally accepted, and the cave would be a biggie.
As to how many people are currently looking, that's a good question, and one I've asked myself. The area is vast and full of potential, they say, but where exactly to look? I walk there regularly and have recently started to turn over some rocks as I go, but it would be interesting to know where the best potential places to dig would be.