Rinse tomatoes under cold running water. Remove wedge-shaped stalk with a sharp kitchen knife. Dry tomatoes with kitchen roll. Prick a few times with a small toothpick. Drain olives well in a colander.
Rinse basil, shake dry and pluck off leaves. Put olives, tomatoes, basil and peppercorns in a jar and fill with olive oil. Cover and leave to infuse for 2-3 days. If kept cool (in the refrigerator or cellar), the pickled tomatoes will keep for about 14 days.