Shell the prawns and set the meat aside. Heat the oil in a pan on a medium-high flame, and fry the shells until pink. Add 1 litre of water and bring to a simmer, then strain and discard the shells.
Return the prawn stock to a clean pan, and add the lime leaves, lemongrass and galangal. Bring to a simmer and leave to infuse for 10 minutes, then add the chillies and simmer for another 5 minutes.
Add the prawns and cook through until pink, then take off the heat and stir in the sugar, lime juice and fish sauce. Taste for seasoning, and then pour into bowls. Garnish with coriander or basil and serve immediately