Add the drained fettuccine to the saucepan with the warm clotted cream mixture. Sprinkle the seasoned Parmesan over the pasta and gently fold everything together using a rubber spatula, keeping the pan over a low heat.
Once combined, splash in 3 tablespoons of the reserved cooking water. The pasta might look a bit wet and loose at first, but keep stirring steadily until the water is absorbed and the sauce transforms into a glossy coating that clings to the noodles.
Taste and adjust the seasoning if needed, then transfer the Alfredo to heated bowls. Garnish with fresh parsley and serve immediately.