Drop tomatoes into boiling water for 1 minute. Drain them and peel off the skin (when they are cool enough to handle). Once they have been boiled the skin comes off very easily. Cut tomatoes into pieces and reserve juice.
Add the tomatoes with juice, chopped carrot, celery and onion to a large saucepan on low heat. Cook uncovered at a slow simmer for 30 minutes, stirring occasionally.
Add the olive oil and garlic, bringing sauce to a slightly stronger simmer for 15 more minutes. Stir frequently and mash tomatoes with the back of a spoon.