Preheat oven to 400F, 200C, Gas Mark 6. Cook the pasta shells according to the manufacturers instructions. Drain. Heat the oil and gently fry the bacon and mushrooms until tender. Meanwhile melt the butter in a saucepan, stir in the flour and cook gently for 1-2 minutes. Off the heat, stir in the milk. Add the Mace, return to the heat and bring to boil stirring continuously to thicken the sauce. Remove from the heat, add the cheese, pasta shells, bacon and mushrooms. Pour into a large ovenproof dish, sprinkle with the breadcrumbs and Parmesan and bake for 25-30 minutes until golden.