1) Preheat the oven to 160C/Gas 3. Bring a large saucepan of salted water to a boil. Add the macaroni and cook until slightly al dente, about 10 minutes. Drain and set aside to keep warm.
2) Whisk the eggs in a large bowl until frothy.
3) Add the Velveeta, butter and 480 ml of the single cream to the large bowl of eggs. Add the warm macaroni, tossing until the cheese has melted and the mixture is smooth. Add the remaining single cream, 3 cups of the sharp yellow Cheddar, the remaining grated cheeses, salt and pepper, tossing until completely combined in the large bowl.
4) Pour the mixture into 23-by-33 cm casserole or baking dishes (approximately three (3-litre) baking dishes) and bake for 30 minutes. Sprinkle with the remaining 1 cup of sharp yellow cheese and bake until golden brown on top, about 30 minutes more.