Lay the fish fillets in a frying pan, pour over just enough milk to cover, then add the bay leaves, lemon rind and salt and pepper. Cover and simmer for 8 minutes or until the fish is just cooked and flakes easily.
Lift the fish out on to a plate, peel away the skin, then break into large flakes, removing any bones. Strain the milk and mix with the remaining milk.
Heat the butter in a saucepan, stir in the flour and cook briefly, then gradually mix in the milk and bring to the boil, stirring. Stir in the fish stock and cook over a low heat for 3–4 minutes. Stir in the sweetcorn, spring onions and cheese, then season generously with salt and pepper. Cover the surface of the sauce with wetted greaseproof or nonstick paper and leave to cool.
Fold the flaked fish and mixed seafood into the sauce, then spoon into 4 individual round pie dishes. Roll the pastry out thinly on a lightly floured surface and cut out 4 pastry lids. Brush the dish rims with egg and press the lids in place. Cut fish shapes from the pastry trimmings and arrange on top, then glaze with egg.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 25–30 minutes until the pastry is well risen and golden and the filling is piping hot.