Put the stock, fish sauce, sugar, lemon grass and galangal or ginger in a large saucepan and bring to the boil, then reduce the heat and simmer gently for 15 minutes. Strain through a sieve to remove the galangal or ginger and lemon grass. Return the liquid to the pan and stir in the chopped coriander stalks.
Add the chicken and simmer gently for a further 4–5 minutes or until cooked through. Add the bamboo shoots, if using.
Meanwhile, put the noodles in a large saucepan of boiling water, turn off the heat and leave to stand for 4 minutes, or according to the packet instructions, until tender. Drain well and divide between deep bowls. Ladle over the chicken soup, sprinkle with the chopped coriander leaves and serve immediately.