Slash along the back of each prawn, then wash and dry them. Mix together the flour, salt and pepper. Dredge the prawns in the seasoned flour and then deep fry them in about 1" hot oil until golden. If using a wok do them in batches. Drain.
Heat 2 tbsps oil in a wok or frying pan. Cook the peppers and chilli together for 2-3 minutes.Add the rice wine or sherry and sweet and sour sauce. Season to taste with the sugar and pepper and bring to the boil.
Mix together the oil, cornflour and water and add to the sauce. Stir until thickened. Toss the prawns back into the pan and heat through,stirring, for 1-2 minutes before serving.