Clean and cut up the leeks and scald them for a few minutes in boiling water. Peel and cut up the potatoes. Drain the leeks. Add the leeks and potatoes to the milk. Add salt and pepper to taste and boil until the vegetables are tender. Melt the butter in a saucepan, and rub in the flour using a wooden spoon. Drain the liquor from the vegetables slowly into the flour and the butter, stirring to keep the mixture smooth. Fry the rashers crisply and grate the cheese. Pour the sauce over the leeks and potatoes and garnish with the bacon. Serve with the grated cheese.