Using a pastry brush, lightly brush a 1.2 litre ring mould with the oil. Place the cod, bay leaf, lemon and peppercorns in a pan. Cover with cold water and bring to simmering point. Poach for 10-15 minutes, or until the fish flakes when tested with a fork.
Meanwhile, carefully line the oiled ring mould with overlapping slices of smoked trout, leaving plenty hanging over the edge. Remove the cod from the pan with a fish slice. Reserve the stock to use for another recipe. Chop the cod into bitesize chunks and put it in a large bowl.
Place 4 tablespoons of cold water in a small heatproof bowl and sprinkle the gelatine over the surface. Leave for 5 minutes, until spongy, then place the bowl over a pan of hot water. Stir until the gelatine has dissolved. Leave to cool slightly.
Add the prawns, sour cream, cream cheese and dill to the cod in the bowl. Pour in the lemon juice and Tabasco sauce. Using a fork, mash the mixture together until well combined. Season to taste with salt and plenty of ground black pepper.
Using a large metal spoon, fold the dissolved gelatine into the fish mixture, making sure it is evenly incorporated. Spoon into the lined ring mould and smooth down the top with the back of the spoon.
Lift the overhanging edges of the smoked trout and fold them over the top of the mousse. Cover the mousse with clear film and chill in the refrigerator for at least 2 hours or until set.
To serve, carefully run a round-bladed knife around the edge of the mousse, invert a serving plate on top and turn both over. Shake the mould and plate together, if necessary, until the mousse drops out on to the plate. Garnish with the herbs and lemon wedges and serve.