After 3 or so minutes, or until the tomatoes have softened a bit, chuck in the prawns. Once it comes back to the boil, and the prawns have turned pink, turn off the heat. You don't want to overcook them as they will continue to cook in the hot soup. Stir in the milk and take off the heat. Add the coriander and lime juice (never cook lime juice, it turns bitter), and check the seasoning, adding fish sauce, or sugar if you think it needs it.