Bring a large pot of salted water to a boil. Add the pasta and cook until al dente according to package instructions. Drain and set aside.
In a large frying pan over medium heat, cook the chopped bacon until crispy. Remove the bacon with a slotted spoon and set aside, leaving the rendered fat in the pan.
Add the sliced leeks to the pan with the bacon fat and cook over medium heat until softened, about 5-7 minutes. Set aside with the bacon.
In a medium saucepan, melt the butter over medium heat. Stir in the flour and cook for about 1-2 minutes until lightly golden, stirring constantly.
Gradually whisk in the milk, ensuring no lumps form. Cook, stirring frequently, until the mixture thickens, about 5 minutes.
In a small bowl, beat the egg. Slowly add about 60ml of the hot milk mixture to the egg while whisking continuously to temper it.
Whisk the tempered egg mixture back into the saucepan with the rest of the sauce.
Stir in the grated cheddar and Parmesan cheeses until melted and smooth.
Add the English mustard and Old Bay seasoning, and season with salt and pepper.
In a large mixing bowl, combine the cooked pasta, cheese sauce, bacon, and leeks. Mix well to ensure everything is evenly coated.
Preheat your oven to 160°C.
Pour the mac and cheese mixture into a greased baking dish (approximately 20cm x 20cm or individual dishes).
Sprinkle breadcrumb evenly over the top.
Bake for 20-25 minutes until the top is golden and bubbly.
Let the mac and cheese cool for about 5 minutes before serving.