Bring a 20cm or 8" saucepan of well-salted water to a boil (I use 1½-2 teaspoons of salt in a pan this size).
Peel the skin off the mackerel and discard it, then break the flesh into flakes.
Peel and mash the garlic into a paste.
Zest and juice the lemon.
Finely chop the parsley.
Add the pasta to the boiling water and cook as per the instructions, noting how long the pasta should take to cook.
Heat a medium (24cm or 10") frying pan over a medium heat, and when it is hot, add one tablespoon of the oil. Add the garlic and breadcrumbs, lemon zest and cook until golden, stirring regularly to avoid burning the breadcrumbs. Once they are golden, remove them and set them aside on some kitchen paper
3 minutes before the pasta is due to be cooked, heat the remaining tablespoon of olive oil over a medium heat. The same pan you toasted the breadcrumbs in is fine. Throw in the chilli flakes and cook for 30 seconds.
Then add the flaked fish. This is already cooked, so it just needs to be brought to a nice temperature, which should take a couple of minutes.
Drain the pasta (reserving 75ml or ⅓ Cup of the cooking liquid) and add it to the fish, toss to coat and add half of the lemon juice and most of the chopped parsley (save a little for garnish).
Add just enough of the cooking liquid to get a nice silky coating in the pasta.
Serve topped with the toasted breadcrumbs, a little bit of the remaining lemon juice and freshly chopped parsley.