Hi,
Pretty sure that the mine you visited was not Tregembo but Tindene.
The photo's are certainly Tindene and there is no public access to Tregembo.
If you look at the 1:25000 OS map, the chimneys and workings in the fields to the north of Tregembo house are the remains of Tregembo Mine, while the workings to the south, on the other side of the lakes, are the remains of Tindene.