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Preheat the oven to 190ºC/ Gas Mark 5. Cook the macaroni in a pan of lightly salted boiling water according to pack instructions, for about 10 minutes. Drain and return to the pan to keep warm.
Meanwhile melt the butter in a large pan, add the bacon and cook over a medium heat, stirring occasionally for 4-5 minutes or until the bacon is coloured. Stir in the flour and cook for one minute.
Gradually stir in the milk, and stir over a medium heat, until the sauce starts to thicken. Stir in the mustard powder and most of cheese, reserving a little for the top. Stir until the cheese has melted, then stir in the ranch dressing and season to taste with salt and freshly ground black pepper.
Stir the drained macaroni into the sauce, then transfer to an oven-proof dish. Sprinkle with the remaining cheese and place the tomatoes on top. Cook in the oven for 15-20 minutes until golden and bubbling.