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Cook the potatoes in boiling salted water for 5-7 mins, until almost tender but firm enough to grate. Drain and refresh under cold running water. Put the coley in a shallow saucepan and pour over the milk. Bring to a simmer and cook for 5 mins. Drain the fish, reserving the milk, then set aside.
Heat half the butter in a medium saucepan, add the leek, then cook for 5-6 mins until softened. Stir in the flour for 1 min, then remove from the heat. Gradually add the milk, stirring well between each addition. Return to the heat and stir until the sauce comes to the boil. Simmer for 2 mins, then stir in the parsley and mustard.
Heat the grill to high. Flake the fish into large chunks, fold into the sauce, then place in a small ovenproof dish. Coarsely grate the potatoes. Melt the remaining butter, toss with the potatoes, season and scatter over the dish. Place under a medium grill for 5-10 mins until the potatoes are golden and tender.