Lay the sardines on a clean kitchen towel, and remove any excess moisture from them. Refrigerate until ready to grill.
Preheat a grill or broiler to medium high, and make sure the grates are VERY clean (to prevent the fish from sticking).
In a mixing bowl, combine the diced tomatoes, 1/2 cup olive oil, olives, parsley, basil, onion, garlic, salt and pepper. Mix well, then set aside at room temp until you are ready to serve.
Place the sardines on a rimmed baking sheet, then drizzle with 1/4 cup olive oil, turning to coat completely. Season with the remaining salt and pepper.
Oil the grill grates well, then place the sardines directly on the grates. Cook undisturbed, until the skin is crispy and slightly charred (2-3 minutes), at which point they will release easily from the grill. Turn them over, and cook for another 2-3 minutes.
While the fish are cooking, brush the lemon slices and tomato halves with some of the remaining olive oil, and season to taste with salt and pepper. Place on the grill with the fish, and cooked until they get char marks, about 2 minutes.
Place the sardines on a serving platter, drizzle with the remaining olive oil and 1/4 cup lemon juice. Serve immediately, with the grilled lemon and tomatoes, and the tomato-olive vinaigrette spooned over the top.