To make the cheese sauce, add a good size dollop of butter to a saucepan and let it melt over medium heat. While stirring slowly pour in the flour and keep stirring. Add a good dash of the milk, then half of the cheese. Quickly stir together and add more milk to achieve your desired consistency.
Now drain the pasta and place in a greased baking dish. Sprinkle the chopped ham and mushrooms on top of the pasta, the pour over the cheese sauce. Sprinkle with the remaining cheese.
Bake in a 190 C / Gas 5 oven for 20 minutes, or until browned and bubbly.