Cook the pasta according to the pack instructions.
Meanwhile, drain the anchovies, reserving the oil. Heat a little of the reserved oil in a small frying pan. Add the anchovies, garlic and chilli, if using, and cook for about 3 minutes until the anchovies have dissolved into the oil. Stir in the tuna and lemon juice to taste, then season with plenty of pepper.
Drain the pasta, reserving a little of the cooking water. Return to the pan and stir through the tuna sauce and most of the parsley. Add some of the reserved cooking water to loosen the mixture if necessary. Scatter with the remaining parsley to serve.