Peel and slice potatoes thinly, and soak in cold water for a few minutes. Drain. Place sliced potatoes in a heatproof gratin dish; add milk; season to taste with salt and freshly ground black pepper, and cook for 20 minutes, or until half done. Drain.
Place drained potato slices in a buttered gratin dish; add cream; sprinkle with freshly grated cheeses; dot with butter and cook in a slow oven (