Bring a pan of water to the boil, add the macaroni and cook for 5 minutes.
Add the broccoli and cook for a further 5-7 minutes, until the macaroni and broccoli are just tender.
Meanwhile, preheat the grill to high. Heat the cheese sauce in the microwave or in a saucepan according to pack instructions.
Drain the macaroni and broccoli, mix with the warmed cheese sauce and tip into an oven-proof dish.
Scatter the halved tomatoes over the top and sprinkle over the grated Cheddar.
Place under the grill for 5 minutes, until the cheese is melted and bubbling and the tomatoes have softened. Divide between 4 plates and serve.