If using raw chicken, put the stock in a saucepan, add the raw chicken and season with salt and pepper. Cover and bring to just below the boil. Skim, then reduce the heat and simmer, covered, for 10–15 minutes for thighs and 15–20 minutes for drumsticks until the chicken is cooked through and the juices run clear when you cut a piece. Remove the chicken from the stock and set aside until cool enough to handle. Meanwhile, bring a separate large pan of salted water to the boil.
Add the carrots, celery and leek to the stock and return to the boil. Reduce the heat and simmer for 10–12 minutes until tender.
Meanwhile, add the noodles to the boiling water and cook for 10 minutes, or according to the package instructions, until tender. Drain, rinse under cold running water and set aside.
Cut the chicken meat from the bones and into bite-size pieces. Add the freshly cooked or leftover chicken to the soup, stir in the noodles and simmer gently to reheat. Adjust the salt and pepper, if necessary, stir in the parsley and serve.