Place the rice noodles in a bowl and cover with boiling water. Leave for five minutes until softened.
In a saucepan, heat the oil and fry the prawn heads, garlic, galangal and chilli for 2-3 minutes.
Add the tomato purée and stock and simmer for five minutes. Remove the prawn heads and discard.
Add the king prawns and coconut milk and simmer for a further 2-3 minutes, or until the prawns are completely cooked through. Drain the noodles and add to the soup.
To serve, sprinkle over the coriander.