Use to visit Ffestiniog one when I was a child and use to take my children when I grew up and lived in North Staffordshire and from memory I think that was pumped in some fasion. Know I live in wales since my 40s through work I often chuckle to myself how its to far t visit for the day know I live in Wales than when I lived in England. I find engineering like these very clever.
From what I understandFfestiniog although built in 1963 can still power the wholes of North Wales for several hours in the event of a total power outage.
The Dinorwig pwer station in Snowdonia, I believe is the the biggest in the UK and over 3 times the size of Ffestiniog.
Did you know @Wigwam new legend has it if your walking Snowdonia and listen carefully you can hear Julie Andrew singing t”sound of water” ?