The day before serving, put the kippers, flesh side down, in a large, shallow dish and pour over the milk and water to immerse them. Cover the dish tightly with foil to prevent the odour from tainting other foods, and place in the refrigerator for 3 hours. (This removes some of the salt from the kippers.)
Drain and rinse the kippers, pat dry with paper towels and place, flesh side up, in a shallow dish. Mix the olive oil in a small bowl with some pepper and the sugar, parsley, spring onions and chives, and spoon over the kippers. Cover the dish tightly with foil and chill in the refrigerator overnight.
Next day, preheat the grill. Place the kippers, skin side up, on a piece of foil on the grill rack and cook them gently for about 5 minutes, or until the skin is crisp and the fish is hot and cooked through.
Serve the kippers, flesh side up, on hot plates with lemon wedges, if desired, and thin slices of brown or rye bread and butter.