Pre-heat the grill.
Heat the oil in a large non-stick pan and fry the chicken for five minutes.
Add the leeks and cook over a low heat for 10 minutes covered.
Meanwhile cook the pasta according to instructions.
Melt the butter in a small pan and stir in the flour to form a roux and cook for 1 minute (a small whisk is useful).
Gradually add the milk off the heat and then bring to the boil stirring until thickened.
Add half the cheese and seasoning and stir until melted.
Put the chicken and pasta in an oven proof serving dish and pour over the sauce.
Sprinkle over the rest of the cheese and breadcrumbs and grill for 2-3 minutes until golden.
Serve with a mixed salad.