Preheat oven to 180 C / Gas mark 4. Place potato slices in a medium bowl with enough water to cover.
Heat oil in a medium saucepan over medium heat. Stir in mince, onion, tomato puree and Worcestershire sauce. Season with salt and pepper. Cook until mince is evenly browned and onions are tender.
In a separate medium saucepan over medium heat, melt 50g butter and thoroughly blend in flour. Gradually stir in milk. Cook and stir 5 minutes, or until thickened. Reduce heat, and blend Cheddar cheese into the mixture. Season with salt and pepper to taste.
Line a medium baking dish with 1/2 the potato slices. Pour in the mince mixture, and top with mushrooms. Cover with the cheese sauce mixture. Top with remaining potatoes. Dot with knobs of the remaining 50g butter.
Bake 30 to 40 minutes in the preheated oven, until lightly browned.