Preheat your oven to 400°F (200°C) and place the peeled, chopped potatoes in a large pot of salted water to boil until tender, about 15 minutes.

While the potatoes cook, place the white fish and smoked haddock in a deep frying pan and pour over the milk; bring to a gentle simmer and poach for 5 minutes until the fish flakes easily.

Remove the fish from the milk with a slotted spoon and place it into a baking dish, reserving the poaching milk for the sauce.

In a separate saucepan, melt half the butter over medium heat and sauté the sliced leeks until soft, about 5 minutes.

Stir the flour into the leeks and cook for 1 minute, then gradually whisk in the reserved poaching milk until you have a smooth, thick sauce.

Remove the sauce from heat, stir in the frozen peas, parsley, and raw prawns, then season with salt and pepper.

Flake the cooked fish in the baking dish into large chunks, then pour the creamy leek and prawn sauce over the top and mix gently to combine.

Drain the cooked potatoes and mash them with the remaining butter and a splash of milk until smooth.

Spread the mashed potatoes evenly over the fish mixture, using a fork to create texture on the surface, and sprinkle the grated cheddar cheese on top.

Bake in the preheated oven for 20-25 minutes until the top is golden brown and the filling is bubbling at the edges.

Let the pie stand for 5 minutes before serving to allow the filling to set slightly.