Cook the potatoes in boiling salted water for 5–7 minutes, until almost tender but firm enough to grate. Drain and refresh under cold running water.
Put the ling and prawns in a shallow saucepan and pour over the milk. Bring to a simmer and cook for 5 minutes. Drain the fish, reserving the milk, then set aside.
Heat half the butter in a medium saucepan, add the leek and cook for 5–6 minutes until softened.
Stir in the flour and cook for 1 minute. 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 minutes and then stir in the parsley.
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 potatoes, season and scatter over the dish. Place under a medium grill for 5-10 minutes until the potatoes are golden and tender