Make the marinade by mixing together the fish sauce, black soy sauce, sugar, cornflour, garlic and oil in a medium sized bowl. Add the pork pieces and turn to coat. Let sit for 15 minutes.
Blend mango juice with 25g mango in a liquidiser or food processor until smooth.
In a medium-sized bowl, mix together the mango/juice mix, water, vinegar, sugar, tomato ketchup, lime juice and fish sauce and set aside.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir-fry shallots and red peppers until fragrant, about 3-4 minutes. Stir in the pork and stir-fry until browned, 3-4 minutes. Reduce the heat to medium. Continue to cook until the pork is fully cooked, 3-4 minutes.
Stir in the sauce and mix well. Once the sauce has come to the boil, turn off the heat. Stir in the diced mangoes and coriander, serve hot.
Black soy sauce is thicker, darker and sweeter than ordinary soy sauce. It can be found in Chinese/Oriental speciality stores or online.