Trim the outer leaves and the end of the stalk from the cauliflower. Cut round the stalk to cut off the florets (halve any large ones). Bring a pan of salted water to the boil and drop the florets into it. Simmer for 8 minutes, then drain.
Cook the pasta, uncovered, in a large pan of rapidly boiling salted water. The water must boil steadily to keep the macaroni moving freely and prevent it from sticking to the saucepan. Leave the lid off to prevent boiling over. Cook the macaroni until it is just tender. Drain well and rinse under hot water to remove surface starch that would make it sticky.
Melt the butter in a second, heavy saucepan and add the flour, chilli flakes and mustard. Cook, stirring, for 1 minute. Remove from the heat. Gradually pour in the milk, whisking until smooth. Return to the heat and stir until boiling. Simmer, stirring all the time, for 2 minutes. Remove the pan from the heat.
Stir in all but 2 tablespoons of the cheese and once melted add the cream and stir. Season the sauce to taste with salt and pepper. Mix the macaroni into the sauce. Turn half the mixture into a shallow ovenproof dish. Lay the pieces of cauliflower on top, and finish with another layer of pasta mix.
Mix the rest of the cheese with the breadcrumbs and sprinkle evenly over the sauce; make sure that all the sauce is covered, or it will form brown blisters under the grill.
If eating straight away, while everything is hot, turn the grill to its highest setting and grill until the top is brown and crisp.
If the dish is for later, refrigerate now and half an hour before serving put it into the middle of a preheated oven set at 200C/180C Fan/Gas 6 until the top is brown (about 20 minutes).
Sprinkle with the caraway seeds before serving.