Cook the cauliflower in boiling water for 15 minutes, until very soft. Drain and purée in a food processor until smooth. Meanwhile, cook the macaroni in boiling salted water for 10 minutes until cooked, then drain.
Preheat the grill to medium high. In a medium-sized saucepan, heat the oil gently, then stir in the flour. Leave over a gentle heat and whisk in the milk gradually. Simmer for 1 minute to thicken. Remove from the heat and whisk in the soft cheese and two-thirds of the grated cheddar. Fold in the cauliflower purée and the cooked macaroni.
Pour into an ovenproof dish, sprinkle over the remaining cheese and the breadcrumbs and grill for 10 minutes, until golden.