You've got it Hugo. If you Google the B4409 as it leaves Tregarth by the Shiloh Chapel crossroads and travel app. 150mtrs west past the Garage/Depot on the left you come to the sign for the Lon Las Ogwen cycle path, also on the left. Look down the track and zoom in to the left of the wooden fencing and you can just see a bit of the toothed shape of the stone on the right of the path. It marks the start of the lower part of the 11 mile cycle path from Ogwen Cottage and follows the old narrow-gauge and also part of the old B.R. Bangor-Bethesda track bed over the new bridge which you identified recently down to the end at Porth Penrhyn