Arrange the tomatoes close together, cut-side up, in a roasting tin. Scatter with the garlic and basil, drizzle with 1 tablespoon of the oil and vinegar and season well with salt and pepper. Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 1½ hours.
Meanwhile, heat a griddle pan over a high heat and griddle the pancetta slices for about 2 minutes, turning once, until crisp and golden. Transfer to a plate lined with kitchen paper until needed, leaving the pan still over a high heat.
Sear the scallops for 1 minute, then turn them over and cook for 1 minute on the other side until cooked and starting to caramelize. Remove from the pan, cover with foil and leave for 2 minutes while you cook the artichoke hearts in the pan until hot and charred.
Toss the lamb's lettuce with the remaining oil and vinegar and arrange on serving plates. Top with the artichokes, tomatoes and scallops, crumble the pancetta over the top and serve immediately.