Thanks Lee for posting the photos and updated plan. This feature is definitely not the outlet to the culvert from Smallpenny Sough (named as an adit in your earlier photos) as that is close to the waterfall which is in the background of your photo (beneath which is the entrance to Lathkill Resurgence Cave, only accessible every few years when the weather is exceptionally dry). There is a vein high on the valley side and crossing the fields above (several shaft mounds, none open) just west of Carters Mill that runs north-south and crosses Gank Hole Vein. There is an open adit up there (low and ends in a forefield, no stopes) and a possible adit higher up at the end of a cutting chocked with impenetrable brambles) - I wonder if your outlet and slabs may be related to drainage of these workings.