Which quarry are you looking at? The one at the back of the pump house has really easy access down to it. I dived it years ago and it has a path down to some rotten wooden steps. Pretty dull dive, flat bottom at about 30m which didn't look much like changing. I plumbed it with a gauge tied on a reel in various places and it was pretty steady 30m all over.
If you mean the other one near the Tal-y-Sarn track then there doesn't appear to be an easy way in. The old tale was that this one was chemically contaminated, don't know if it's true, the access put me off more.
I abseiled into the pumping shaft and dived there on a pair of 3's in 1999 if you're looking for some "vertical" access
