Place the cod fillet in a medium, heavy-based frying pan and pour over the measured water. Bring to the boil, reduce the heat, cover and simmer for 3 minutes until the fish is opaque and cooked through. Drain and reserve the fish stock. Flake the fish into chunks and gently toss with the prawns.
Cook the carrot and broccoli in boiling water for 5 minutes, drain and set aside. Melt the butter in a pan, remove from the heat and add the flour. Stir over a gentle heat for 30 seconds. Remove from the heat and gradually add the milk a little at a time, stirring well after each addition, then add the reserved fish stock and stir well.
Return the pan to the heat and bring to the boil, stirring continuously until thickened. Remove from the heat and add the Cheddar. Pour over the fish, add the vegetables and very gently fold together. Transfer to a large gratin dish and set aside.
Meanwhile, cook the sweet potatoes for 8–10 minutes until tender. Drain, add the butter and mash well. Stir in the parsley, spoon over the top of the fish and sauce and sprinkle with the Cheddar. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 30 minutes until a golden crust forms.