If frozen, defrost the spring roll skins thoroughly. Soak the mushrooms in warm water for 20 minutes. Squeeze out the excess liquid. Remove and discard the stems and finely chop the caps. Soak the noodles in a large bowl of warm water for 15 minutes. When soft, drain and discard the water. Cut into 3 inch lengths.
Put the prawns and pork in a large bowl, add salt and pepper and mix well. Then add the rest of the ingredients except for blanched green onions and frying oil, noodles and mushrooms and stir them well into the prawn and pork mixture. Wrap the bowl with plastic wrap and chill it for at least 20 minutes.
For the sauce, mix the cornstarch and water. Place all the ingredients for the sweet and sour sauce, except the cornstarch mixture and chili in a pan. Bring to a boil, stir in the cornstarch mixture and cook for 1 minute. Stir in the chili flakes. Let cool and set aside.
When you are ready to stuff the dumplings, put 2 tablespoons of the filling in the center of each spring roll skin. Dampen the edges with a little water and bring up the sides of the skin around the filling. Pinch the edges together at the top so the dumpling looks like a small, filled bag. Tie with a green onion. Continue, to use up all the filling.
Heat a wok over high heat. Add the oil and, when it is moderately hot, add a handful of dumplings and deep-fry for 3 minutes until golden and crispy. If the oil gets too hot, turn the heat down slightly. Drain on paper towels and fry the remaining dumplings. Serve immediately with the sweet and sour sauce.