Arrange a quarter of the pancetta slices in a row on a board, overlapping them slightly along their long sides. Put a chicken breast at one end so that it lies across the slices of pancetta, top with 3 sage leaves and sprinkle with a little sea salt and freshly ground black pepper. Wrap the chicken in the pancetta so that it is completely enclosed. Repeat to make 4 parcels altogether.
Spoon the oil into a large heavy pan (a cast-iron sauté pan is ideal, or you could use a roasting tin). Put the pan on the barbecue and heat for 1–2 mins until hot, then lay the chicken parcels in the pan and cook for 7–8 mins on each side. The pancetta will become golden and crispy.
Remove the parcels, lay the tomatoes in the pan and shake to coat in the cooking juices, then heat through for a couple of minutes. Return the chicken to the pan, take the pan off the heat and leave the chicken to rest in it for about 10 mins, so it will cut more easily.