hi pete welcome to an.
do you recall anything else? a place it was near?
have you looked at the photos on the site?
it may have been dinorwig as the cables ties in quite well but could have been some others. have a look at penyrorsedd possibly.
dinorwic has a footpath running right through the middle and looks down upon llanberis and can be visited. it is near the national slate museum.
penyrorsedd is a working quarry so no official access ;-)
i'm sure other will have other suggestions