Place a large saucepan of salted water over a high heat & bring to the boil. Once boiling, add in the macaroni then cook to al dente. This will take around 5 minutes.
Refresh the macaroni under cold water then drain in a colander. Coat the pasta in a small amount of olive oil then set aside.You can cook the macaroni in advance & store in the fridge until needed! Next, grate the cheddar & red Leicester and cut the mozzarella into small pieces. Set aside for now.
Add the butter into another large saucepan then set over a medium heat & cook until melted. Stir the flour into the melted butter then cook out for a minute, stirring constantly.
Add in half the milk then whisk until smooth. Cook out until thick, whisking/stirring regularly. This will take 2-3 minutes over a medium heat.
Pour in the remaining milk & keep cooking until thickened again.
Now add in half the chicken stock & reduce the heat to medium-low. Continue cooking, stirring/whisking regularly until the sauce has thickened back up.
Pour in the rest of the stock then keep cooking until the flour has been cooked out & the sauce has thickened for a final time. It should be slightly thicker than double cream.
Add the mustard & all of the cheese into the sauce then stir to melt. Season with salt & pepper then take off the heat & stir in the macaroni.
Tip the mac & cheese into a baking tin, cover the surface with a piece of clingfilm or baking parchment (this stops a skin forming) then leave to cool completely.