Heat oil in a large heavy-based saucepan over medium-high heat. Add chicken pieces and cook, in batches, until browned all over. Remove and set aside. Add onion, celery, carrot and pancetta to pan and cook over low heat for 5 minutes until onion softens. Add garlic and mushrooms and cook for a further minute.
Return chicken to pan, add wine and simmer 1-2 minutes. Add sauce, bay leaf, vinegar, rosemary and stock. Bring to the boil, then reduce heat to low and cook, covered, for 20 minutes, stirring occasionally. Add olives and cook for a further 10 minutes.
Remove chicken and bay leaf, then reduce sauce over high heat for 5 minutes. Divide chicken and sauce among 6 plates and serve topped with parsley.