Looks like Dinorwig to me, part of Electric Mountain power station.
Merddin's got it, I'm afraid, Yorkie! Its official name is the Dinorwig Cable Tunnel, originally part of Dinorwig Quarry but used now to carry the 400,000 volt cables that transfer the electricity generated by Dinorwig Pumped Storage Power Station to the National Grid.
Dinorwig Power Station, located adjacent to the Snowdonia National Park in Gwynedd, North Wales, is Europe’s largest pumped storage hydroelectric power station. It is also one of the fastest, most dynamic power plants in the world, capable of delivering its full station output of 1800 MW in only 12 seconds.