Way to go Skiff!

It's a Gasholder or sometimes Gas Holder, because that's what it does. A gasometer is a device that measures gas, some of which look like small gasholders. The public acceptance of the wrong term is due to the press who often manage to get things wrong.

If you're interested, Fulham Gasworks, claims to have the oldest gasholder in the world. 1860 something, I think.

Bert.