Preheat the oven to 200°C/400°F/Gas mark 6.
Put the cod and salmon into a saucepan with 300ml/10fl oz of the milk and the bay leaf. Season with salt and pepper. Bring to the boil, cover and simmer gently for 10–15 minutes until tender.
Cook the potatoes in salted boiling water for about 15 minutes until tender.
Drain the fish, reserving the cooking liquid. Flake the fish, discarding any bones, and put in a shallow ovenproof dish. Add the prawns.
Melt half of the margarine in a saucepan, add the flour and cook, stirring, for 1 minute. Gradually stir in the reserved cooking liquid and the wine, and bring to the boil. Add the dill, capers and salt and pepper to taste, and simmer until thickened. Pour over the fish and mix well.
Drain the potatoes and mash them, adding the remaining margarine and the remaining milk. Season with salt and pepper.
Spread the mashed potatoes over the fish, and cook in the oven forabout 25 minutes or until piping hot and browned. Serve immediately