For the chicken mince sauce, put the oil, celery, onion, carrot, garlic, and pancetta in another large saucepan. Gently cook together until the veg is soft but not coloured. Stir in the chicken mince breaking it up into small pieces. Stir in all the stock, tomatoes & oregano bringing it to a simmer. Cover and gently cook for 1 hour, stirring occasionally. Uncover and gently simmer for another 30 mins-1 hour. Taste and season as required.