Cook the spaghetti in a large pan of boiling salted water according to pack instructions. Meanwhile, make the sauce. Heat the oil in a medium pan and add the chilli flakes, tomatoes and sardines, breaking up roughly with a wooden spoon. Heat for 2-3 mins, then stir in the olives, capers, garlic oil and most of the parsley. Mix well to combine.
Drain the pasta, reserving a couple of tbsp of the water. Add the pasta to the sauce and mix well, adding the reserved water if the sauce is a little thick. Divide between 4 bowls and sprinkle with the remaining parsley.