Place the fish in a saucepan and pour over the milk. Bring slowly to the boil then reduce the heat and simmer for 5-6 minutes until tender. Remove the fish and flake into a bowl, discarding any skin and bones. Reserve the milk. Melt the butter, stir in the flour and cook for 1 minute, stirring occasionally. Gradually stir in the milk and cook for 1-2 minutes until thickened and smooth. Stir in the mascarpone, prawns, lime juice and 1 tablespoon of the chopped parsley and seasoning to taste. Meanwhile, cook the pasta following the instructions on the pack. Quickly drain then stir into the fish mixture. Transfer to a warmed serving dish, sprinkle over the remaining parsley and serve immediately garnished with lime wedges.