Fifty years ago, Ben Cruachan in Argyll and Bute was the equivalent of London’s Silicon roundabout – a place of technological innovation on which an entire nation was pinning its hopes for the future. The madcap idea was to turn a rugged peak in the Scottish highlands into a geological battery: using excess energy in the national grid, water would be pumped from Loch Awe at the bottom of the mountain into a reservoir at the top.