In a large bowl, place potatoes. Sliver dried tomatoes and reserve 1/2 cup of the tomatoes for the dressing. Add slivered tomatoes to potatoes with leeks and toss gently.
For dressing, in a food processor or blender, place reserved tomatoes, yogurt or sour cream, milk, basil, dill, garlic, pepper and salt, if desired. Cover and process until smooth. Spoon onto salad and toss gently to coat.