Put the stock into a saucepan with the lime leaves and lemon grass, bring to the boil, then reduce the heat and simmer for 10 minutes.
Add the noodles to the stock and cook according to the packet instructions. After 2 minutes, add the peas, sweetcorn, prawns, spring onions and soy sauce and cook for 2 more minutes. Remove and discard the lemon grass. Serve the soup in warmed bowls.