Put the potatoes whole into a large saucepan or small pot, cover with water and bring to a simmer. Cook for 20-30 minutes, until just tender when poked with a knife. Pull them out of the water and set aside until they're not steaming hot. Meanwhile, heat the butter, maple syrup and orange zest in a small saucepan until melted and smooth.
Cut them, with their skins on, into thick slices or lengthwise into quarters. Brush with oil and cook on a preheated grill, turning with tongs, for a few minutes, or until char-marked (and heated through if you cooked them ahead). Serve the grilled potatoes warm, drizzled with maple butter.