Mix the cornflour with the fish sauce and sugar, then gradually add 8 tbsp water until smooth. Cook the noodles in a pan of boiling water for 4 mins.
Meanwhile, heat the oil in a wok, then stir-fry the pepper, garlic and spring onions for about 3 mins. Tip in the chicken, spices and peas, stir-fry for a couple of secs more, then pour in the fish sauce and flour mix. Stir until thickened, then toss in the drained noodles and basil or coriander.