Cook potatoes in boiling salted water until just tender, then drain and cool. Cut into 1cm-thick rounds. Cook beans in boiling salted water until just tender, drain and refresh in iced water, then drain again. Combine potatoes, beans, anchovies, onion, caperberries and olives in a large bowl. Place vinegar and olive oil in a separate bowl, and season to taste with sea salt and freshly ground black pepper.