Put the milk in a saucepan, then simply add the flour and butter and bring everything gradually up to simmering point, whisking continuously with a balloon whisk, until the sauce has thickened and becomes smooth and glossy
Then turn the heat down to its lowest possible setting and let the sauce cook very gently for 5 minutes, stirring from time to time. Meanwhile, taste and add seasoning.
Cook the leeks gently until almost tender. Cook potatoes in the same way. Drain and reserve. Layer the potatoes and leeks alternately in a buttered dish and cover with the white sauce. Bake in the oven for around 40 minutes.