Cook the pasta in a large pan of salted boiling water to pack instructions. Drain and toss in oil to prevent it from sticking. Set aside.
Cook the ready-made pulled pork to pack instructions, then shred with two forks and set aside.
Preheat the oven to 200°C (180°C fan) mark 6. Melt the butter in a large pan, add the flour and whisk until smooth, and then cook for 2min on a low heat. Take off the heat and add the milk in several additions, whisking to make a smooth mixture after each addition. Put the sauce back on the heat, bring the sauce to a simmer and cook for 4-5min, whisking constantly until thickened. Remove the pan from the heat and stir through the mustard, Gruyere, Cheddar and 100g (3½ oz) of the mozzarella, season well.
