Pour the stock into a saucepan, add the curry paste, lime leaves, fish sauce to taste, spring onions and mushrooms. Bring to the boil and simmer for 5 minutes.
Bring a separate pan of water to the boil, add the noodles and cook for 3 minutes.
Add the remaining ingredients to the soup and cook for 2 minutes until piping hot.
Drain the noodles, rinse with fresh hot water and spoon into the base of 4 bowls. Ladle the hot prawn broth over the top and serve immediately with small bowls of Thai fish sauce and dark soy sauce for seasoning, if liked.