Preheat oven to 200C.
Melt half the butter in a large saucepan and cook the leeks for 2-3 minutes until soft. Stir in the flour and cook for 1-2 minutes. Remove from the heat and gradually stir in the milk. Return to the heat and cook, stirring until the sauce has thickened. Simmer gently for 5 minutes. Stir in the prawns and season with salt and pepper.
Skin the haddock portions, if you are using portions from the head end of the fish, use some tweezers to remove pin bones. Tail pieces will be free from bones.
Dice the haddock into random-sized pieces. Place half the sauce in a pie dish, add the fish, prawns and any other seafood you like, and spoon over the remaining sauce. Top with the sliced potatoes and dot with the remaining butter. Season with some more pepper.
Place the dish on a baking sheet and bake for 35-40 minutes until the potatoes are golden.