Make up the bread mix according to the packet instructions, kneading until smooth. Preheat the oven to 200°C/fan180°C/gas 6. Roll out the dough on a lightly floured surface to a 40 x 30cm rectangle.
Scatter over the peppers, tomatoes, mozzarella, olives and basil and drizzle with 1 tablespoon of the reserved oil from the tomatoes. Roll up the dough quite tightly from a short end until the filling is enclosed. Brush a large baking sheet with oil and arrange the loaf seam-side-down on top. Leave in a warm place for 15 minutes to rise slightly. Push a skewer right through the centre of the loaf to the baking sheet to make a hole for steam to escape during cooking. Brush with more oil from the tomatoes and scatter with rock salt and coarsely crushed black peppercorns.
Bake in the oven for 35-40 minutes, until risen and golden brown. Remove the bread from the oven and set aside on a wire rack for 15 minutes to cool slightly. Serve the bread warm or cold, cut into slices.