A guitar with one string? There is indeed such a thing. It’s played by Seasick Steve, and it consists of a stubby plank of wood, a pick-up and a couple of nails. And a string. The man born 70 years ago as Steven Wold plays it with a slide, and it makes a fabulous, sleazy sound. It’s one of a collection of manky-looking instruments played by Seasick Steve, the former hobo, drifter, session musician and studio engineer who has experienced a late blossoming in popularity as a bluesman and raconteur.