Put the chicken in a large pan with two litres of cold water. Place over a high heat and bring the water to the boil then reduce the heat to a simmer. Skim away any residue that has risen to the surface of the water.
Add the diced vegetables, the onion quarters and the bay leaf. Season with salt and pepper. Cook and a low simmer for 1 1/2 to 2 hours until you have a lovely tasty stock.
Strain the stock into a separate pan. Reserve the chicken and set aside but discard the vegetables. Place the stock on a medium heat, add the pearl barley and sliced leek and allow to simmer for 25-30 minutes until the barley and leeks are tender.
While the barley is cooking shred the chicken meat, discarding the bones and skin. Add the meat back into the soup once the barley and leeks are cooked.
To serve, divide the chopped prunes equally between your serving bowls. Ladle over the soup making sure the chicken and leeks are equally divided between each bowl. Serve immediately.