Preheat your oven to 450 degrees F (225 degrees C).
Add the butter to a medium pot and melt over a medium heat. Add the flour and cook for 2 minutes, stirring often, to toast the flour slightly.
Gradually whisk in the milk until you have a smooth sauce. Cook until the sauce is gently steaming and has thickened slightly (it will still be quite thin). Remove from the heat.
Mix together all of the grated cheese and set aside a couple of large handfuls of the cheese mixture for sprinkling on later. Take the remaining grated cheese and add to the pot of sauce. Stir until the cheese has melted.
Add the lemon juice/vinegar, marmite (or miso/soy sauce), mustard, smoked paprika and MSG (if using) into the sauce. Taste and season with salt, as needed (remember that once the cooked pasta is mixed in it will taste less salty).
Cook the dry pasta in a large pot of boiling water until al dente (usually 2 minutes less than instructed on the packet). Drain, and tip into a deep baking dish (I use a 9 x 13 inch metal roasting dish).
Pour the hot sauce over the cooked pasta and stir together to coat. Sprinkle the reserved grated cheese on top followed by the breadcrumbs. Drizzle with the olive oil and season with black pepper.
Bake for 15-20 minutes until the topping has browned and the mixture is bubbling.