Cook the potatoes and swede in boiling, salted water until tender. Drain, mash well, and set aside.
Meanwhile, put the salmon, onion, carrot, celery, bay leaf and mustard seeds in a large frying pan. Add the milk, cover and set over a high heat. Just before it boils, reduce the heat and poach gently for 10 minutes or until the fish is opaque.
Lift the fish out of the cooking liquid and set aside. Discard the liquid and flavourings.
Remove any skin or bones from the fish and flake the flesh into large chunks. Place in a mixing bowl and gradually stir in the mashed potato and swede, together with the chives. Add Worcestershire sauce, salt and pepper to taste. Shape the mixture into 8 fish cakes and place on a tray. Cover and refrigerate for 1 hour.
Meanwhile, heat the oven to 190°C (375°F, gas mark 5). Put the egg and 2 tablespoons of water in a shallow bowl and beat. Put the breadcrumbs in another shallow bowl.
Dip the chilled fish cakes in the beaten egg to coat, then in the breadcrumbs. Transfer them to a nonstick baking sheet and brush lightly with oil. Bake for 20-30 minutes or until crisp and lightly browned. Serve with green beans and pea sauce.