Wrong quarry mikem. To be honest, that's the one I immediately thought of when I started reading the post but the one Babyhagrid is talking about sounds like it's just to the North of Cardiff.
There are at least a couple of quarries there Babyhagrid, which one do you mean? There's the one overlooking the junction on the A470, that has lots of climbing routes on it but I'm assuming you mean the one along the road that leads passed the Black Cock and Mountain View Ranch? If that's the one, I've driven passed many times and looked at the Keep Out signs and thought it would be worth a stealthy wander around one day but never did it. Haven't heard of anyone else looking in there but surely cavers have!?