Cook the cauliflower florets in boiling water for 10 minutes. Meanwhile, cook the potatoes in a separate saucepan of boiling water for 10 minutes. Drain both vegetables and put aside until needed.
To make the sauce:
Melt the butter in a large saucepan, add the leek and garlic and cook over a low heat for 1 minute. Stir in the flour and cook for 1 minute. Remove from the heat, gradually stir in the milk, 55g/2 oz of cheese, paprika and parsley. Return to the heat and bring to the boil constantly stirring. Taste and season if needed. Transfer the cauliflower to a deep, ovenproof dish. Add the cherry tomatoes and top with the potatoes. Pour over the sauce and cover completely, sprinkle over the remaining grated cheese.
Cook in the oven fro 20 minutes, or until the vegetables are cooked through and the cheese is golden brown and bubbling. Garnish with chopped parsley and serve immediately.