Place the salmon fillets in a frying pan and pour over enough water to cover. Bring to the boil, then reduce the heat and simmer for 10 minutes or until the fish is cooked through and flakes easily. Remove the skin and any bones and use a fork to break into large chunks.
Meanwhile, heat the butter in a small saucepan, add the leeks and cook over a gentle heat for 10 minutes or until softened and cooked through. Stir in the lemon juice and most of the rind.
Cook the pasta in a large saucepan of salted boiling water according to the pack instructions until al dente. Drain, reserving a little of the cooking water, and return to the pan. Stir through the salmon and leeks and season, then stir in the soured cream, adding a little cooking water if needed to make a sauce. Serve immediately with plenty of black pepper and the remaining lemon rind scattered over.