Cook the leeks for 6-8 mins in boiling water or until just tender. Drain and transfer to an oven proof dish so the leeks fit snugly.
Melt the butter over a low heat and add the plain flour, stirring continuously with a whisk to make a paste. Add the milk slowly, increase the temperature and keep stirring until the sauce thickens then stir in the cheese. Take off the heat and add the Original Wiltshire Mustard.
Pour the cheese sauce over the leeks and sprinkle the breadcrumbs and a little extra cheese over the top.
Put under a hot grill for 5-10 mins or until the breadcrumbs have browned and the cheese has melted.
Serve with a crunchy green salad.