Make 4 lengthways cuts in each leek with a sharp paring knife, starting 3 cm above the root end. Soak the leeks in several changes of warm water until thoroughly clean. Pat dry.
Heat the oil in a large nonstick frying pan over moderate heat. Add the leeks, turning until well coated. Add the garlic and cook for 1 minute or until tender.
Add the tomatoes, 1/2 cup water, the olives, parsley and basil, and bring to the boil. Reduce to a simmer, cover and cook for 10 minutes or until the leeks are fork-tender. Serve warm or at room temperature.