Bring a large saucepan of salted water to the boil, and cook the macaroni by the instructions on the packet, undercooking for 2 minutes. Drain and refresh under cold water after cooking and set aside in an ovenproof baking dish.
In a medium heavy based saucepan, melt the butter. Once melted, add the flour and cook out the paste for 2 minutes. Add the mustard, cayenne and marmite and cook for a further minute.
Add the milk slowly, stirring well after each addition to ensure you have a lump free sauce. Once all the milk has been incorporated, bring the mixture to the boil for 1 minute.
Turn off the heat, add the cheese and stir well until fully melted and incorporated. Season to taste.
Pour the cheese sauce over the macaroni in the baking dish and mix well. Set aside.
Sprinkle the crushed Twiglets on the top and bake for 10-15 minutes until the top is golden brown and bubbling.
