Graham thanks for your very quick reply. Either end would do.
I remember seeing a picture taken by one of our staff and it really is a very impressive tunnel! The Environment Agency fitted a grille over the lower end where it joins the River Ogwen, but the salmon and sea trout take the easy option when the river is in spate and swim up into the quarry pool. On a calm day you can see 30-40 fish. I did catch a sea trout in the pool a couple of years ago! A colleague is planning to bring his boat so we can try for a fish and yes we do both have EA licences.