Preheat the oven to Gas 5, 190°C, 375°F. Place the leeks in a large roasting tin and drizzle with half the olive oil. Season with salt and black pepper. Roast for 15 minutes.
Meanwhile, carefully loosen the chicken skin with your fingers and rub the cream cheese between skin and flesh. Sit chicken legs in the roasting tin on top of the leeks. Surround with the halved tomatoes.
Scatter tomatoes with the garlic and a sprinkle of sugar (bringing out their natural sweetness). Tear the thyme sprigs over the chicken and season the whole dish with salt and pepper.
Drizzle with the remaining olive oil and roast for 45 minutes, or until the chicken is cooked with no pink showing. Pierce flesh with a knife; if the juices run clear the chicken is ready.
Remove from oven, and rest chicken for 5 minutes. Serve topped with the delicious sauce created by the melted cheese.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.