Toss the chicken, leeks, and oil together on a baking tray. Cook for 20 mins until the chicken done through.
Cook the pasta to the pack instructions and drain.
Mix 2tbsp of the milk with the cornflour. Melt the butter in a pan, add the rest of the milk, and bring to the boil. Whisk in the cornflour mix and cook for 2 mins to thicken. Take off the heat, add the cheeses and cayenne and season with black pepper. Let the cheese melt, then stir to combine.
Toss the macaroni and cheese sauce through the chicken and leeks, sprinkle on the breadcrumbs and thyme and bake for 15 mins, until golden and bubbling.