You could contact MacAlpines the local quarries in Bleanau will probably refer you to Head Office (London??) and they'll know nothing about the quarry or refuse point blank because if they give permission they become liable for any injuries or deaths. Try contacting the Caernarfon Slate Company as there was talk of them reopening the quarries in 2005/6 But it sort of went to the wall when MacAlpines rolled in. They may still hold the mineral/access rights.