Make the chicken broth for the soup by placing the chicken carcass, salt, pepper, garlic, bay leaf (if using) and water into a large pot. Bring to the boil and then reduce the heat and allow it to simmer for at least 60 minutes.
Once the broth is ready, sieve the broth, making sure to catch the liquid in a bowl. Discard the chicken bones.
Return the chicken broth to the pot and add the carrots, leek, celery, spaghetti and chicken. Stir in leftover gravy (if using), chicken stock cubes and mixed herbs. Simmer the soup for 10 – 15 minutes, until the spaghetti is cooked. If you find the soup is too thick, you can add more water.
Season the soup with salt and pepper before serving.