Heat oil in a large saucepan over medium heat. Cook onion, celery, garlic and bacon, stirring, for 6-8 minutes until onion is soft and bacon is crisp. Stir in tomato paste and cook for 1 minute. Add potatoes, chopped tomatoes, stock and sugar, season with salt and pepper, then simmer for 10 minutes or until potatoes are tender and the sauce is slightly thickened. Add beans and simmer for 3-5 minutes until heated through.