Heat oil in a large saucepan over medium heat. Add onion, carrot, bacon and celery. Cook for 10 minutes or until onion has softened. Add tomato, zucchini, stock powder and water. Bring to the boil. Add barley. Reduce heat to low. Simmer, partially covered, for 40 minutes or until barley is tender. Garnish with chopped celery leaves, if you like. Stir into soup. Serve.