Cook the noodles in a large pan of boiling water for 3–5 minutes, until tender. Drain thoroughly and set aside. Peel off the tough outer layers of the lemon grass, leaving the tender whitish centre. Cut it into 7.5cm (3in) pieces and crush with the flat of a heavy knife. Shred the chicken into strips. Heat a large, heavy pan over a high heat. Add the oil and, when it is very hot and slightly smoking, add the onion, garlic and lemon grass and stir-fry for about 3 minutes. Stir in the stock and coconut milk, reduce the heat to low, then cover and simmer for 10 minutes. Add the chillies, chicken, fish sauce, soy sauce, sugar, curry paste or powder, salt and pepper and stir well. Add the drained noodles, then cover and simmer for 5 minutes. Remove the lemon grass with a slotted spoon. Stir in the lime juice, then pour the soup into a large tureen, garnish with the coriander and basil leaves and serve at once.