1. Boil the pasta in salted water for 8 minutes, or follow pack instructions. Meanwhile, heat 2 tbsp oil in a frying pan over a medium heat and add the leeks and some seasoning. Sizzle for 5 minutes, stirring often, until softened and turning golden here and there. Remove to a plate.
2. Heat the remaining oil, turn up the heat and wait a few seconds for the pan to get really hot. Add the prawns and tomatoes with a pinch of salt and pepper. Fry for 2-3 minutes until the prawns turn pink all the way through. Add the chilli flakes, garlic and lemon zest, cook for 1 minute more, then add the leeks back to the pan. Turn the heat down.
3. Save a cup of the pasta cooking water, then drain. Tip the pasta into the leek mix, add 5 tbsp of the cooking water, the juice from half the lemon and the parsley and toss well. The pasta shouldn’t be dry – add more of the water if needed. Season to taste, then serve in bowls with the rest of the lemon cut into wedges, for squeezing.