Preheat oven to 450°F. In a large ovenproof (cast iron works great!) skillet, cook the bacon until crisp. Set aside, pouring off all but a couple spoonfuls of the drippings. Cook the leeks in the pan for 5-8 minutes, until soft and turning golden.
Meanwhile, whiz together the eggs, milk, flour, mustard, sugar, salt and pepper in a blender or food processor until smooth.
Scatter bacon over leeks in the skillet and pour the batter overtop; sprinkle with cheese. Bake for 10-15 minutes, until slightly puffed and golden. Cut into wedges and serve warm.