Ffestiniog Pump Storage
There's another (smaller - only 360MW) pump storage in North Wales at Ffestiniog (http://www.fhc.co.uk/ffestiniog.htm), not far from Dinorwig. It's an older design that uses separate pumps and turbines, rather than the reversible pump/tubines like Dinorwig.
The history of both of these are tied in with the nuclear power stations in the area. Ffestiniog was built to complement the Trawsfynydd station, and Dinorwig to complement the Wylfa station on Anglesey.