yes, it's Dorothea quarry.
the issues surrounding the site are extremely complicated. i struggle to keep up with the legal side myself. yes, the owner has said that diving there is banned, but a public right of way passes right next to the quarry (the old road between Talysarn and Nantlle), and so the public (including divers) have access to the site. Furthermore, the local farmer, who is diver-friendly wants to keep access open, so he has access to his land which surrounds Dorothea... still with me? :duh
i dont know what the situation is with insurance to be honest with you. BSAC (and some other organisations) have said they dont want their members to dive there, but i dont know what that means in terms of insurance.
there were 3 deaths last winter which gave Dorothea its unfortunate nickname 'pool of death', given by the local newspaper. the problem is that it's a great place for scuba divers to 'push their limits' and many dive there with totally inadequate skills and equipment and have no respect for the place. among the things i've seen include a solo scuba diver decompressing after diving to over 40m with a single 10 and no redundancy (in 4/5oC water). on one occasion i had to rescue a scuba diver who had made a buoyant ascent from 50m. i had to tow him in with my monofin because he was freaking out and incapable of helping himself.....
the site has never really closed as such, and it's currently in a fairly stable state in that there has been no interference from the owner, since his business ventures were exposed by a regional TV documentary. so noone has permission to dive there as such.
as for insurance, you have to make your own mind about that.
PS: it's now generally accepted that the 'dive centre' promised by the owner will almost certainly never materialise, at least whilst he is the owner. this is no bad thing in my opinion. i would hate to see the place turn into somewhere like Stoney Cove :yack
... so there you have it in a nutshell...
alun