Wash the chicken, and place it in a large stock pot. Add just enough water to cover, and bring to the boil. Skim off any scum that rises to the top.
Then add onion, carrot, bay leaf, and the green parts (top tough parts roughly chopped into fairly large pieces) of the leeks to the chicken and water. Season with a pinch of salt and pepper, lower the heat, cover and simmer for 2 hrs.
After 2 hrs, remove the chicken and vegetables from the stock pot, leaving only the broth. Add the barley (if using) and cook in the broth for the amount of time per package directions. PLEASE NOTE: refer to the package directions when cooking the barley. You may need to bring the broth back to the boil and then reduce heat to a simmer until the barley is entirely cooked. I am assuming cooking time for your barley may range from 15-45 minutes.
Add the tender white parts of the leeks (cut into rounds as in the photo) for the last 15 minutes of cooking. At this time add the cayenne pepper and mace. Taste and adjust seasoning to your preference by adding salt and/or pepper.
In the meantime, remove the chicken from the bones. Add to soup bowls, then ladle the hot soup over the chicken. Garnish with prunes and ENJOY!