incandescent lights not *that* bad?

It uses around 30-35% more power than it generates to get all the used water back up to the top lake.

I believe they have a long term fixed contract with one of the conventional generators for overnight pumping demand rather than buy at the marginal price. The contracts for system support ensure pumped storage is a profitable and stable business.

There was talk a while ago of a new pumped storage scheme being under consideration somewhere in Scotland. There is also an "AC Battery" that was undergoing large scale trials at a Midlands power station site a couple of years back, not sure what the latest news on that is.

I suspected the mechanical losses would be significant. The only long term economic way is probably down to mother nature lifting the water up to the high level resevoir of any hydro set

Probably goes back to the days when there was only one "CEGB"

I hadn't heard about that. I'll see what info I can dig up.