Place chicken breasts and stock in a large stockpot over medium heat and bring to the boil. Reduce heat and simmer for about 20 minutes, or until chicken is cooked through. Remove chicken from stock and set aside to cool.
Add leeks and carrots to stock and simmer for 10 minutes, or until tender.
Shred cooled chicken and return to pot. Add cabbage and egg noodles and cook for another 5 minutes, or until noodles are soft. The soup should be thick like a stew. Serve hot, seasoned to taste with Thai chilli sauce.