Cook the macaroni for about 10 minutes or according to packet instructions until just tender. Add the peas to the pan and cook for a further 2 minutes. Drain and put on one side.
Melt 40 g (1½ oz) of the butter in the rinsed and dried saucepan. Add the flour and stir it in with a wooden spoon. Cook over a gentle heat, stirring, for 1 minute. Remove the pan from the heat and gradually pour in the milk, whisking well. Return the pan to the heat and cook over a gentle heat, stirring continuously until the sauce is thickened and smooth.
Add the mustard, Cheddar and ham and stir until the cheese has melted. Tip in the macaroni and peas and stir until coated in the sauce, then pour into a shallow heatproof dish.
Melt the remaining butter in a small saucepan and stir in the breadcrumbs until they are coated. Sprinkle the breadcrumbs over the macaroni and cook under a moderate grill for about 5 minutes or until golden, watching closely as the breadcrumbs will brown quickly. Use oven gloves to remove the dish from under the grill, and serve.