Place the fish fillets in a shallow pan with the wine, bouquet garni, lemon rind and just enough water to cover the fish. Poach, covered, over a gentle heat for 8—10 minutes until just tender.
Remove the fish with a slotted spoon, drain, flake and cool. Reserve the cooking liquid.
Soften the two cheeses and beat them together until smooth. Place 4 tablespoons of the hot fish liquid in a cup and sprinkle the gelatine over it. Stir until the gelatine is dissolved. Allow the gelatine mixture to cool, then stir it into the cheese mixture.
Whisk the egg whites until stiff and fold into the cheese mixture. Divide the mixture in half. Into one half gently stir the cooked fish, lemon juice, chives and pepper. Pour this mixture into a wetted 1 kg/2 lb loaf tin and chill until set. Into the remaining cheese mixture stir the prawns, tomato purée, parsley and Tabasco sauce. Pour on to the fish mixture in the tin and chill until set. If you are not serving the terrine immediately on setting, cover and chill in the refrigerator.
To turn out the terrine, dip the loaf tin into a bowl of hot water for a few seconds, then invert on to a dish. Serve the terrine in slices, garnished with cucumber slices, whole prawns and dill sprigs.