Cook the bucatini in a large pan of boiling salted water until al dente. Meanwhile, finely slice the garlic and chilli, then roughly chop the anchovies and tomatoes. Heat 3 tablespoons of oil (use the oil from the tin of tuna for added flavour, if you can) in a frying pan over a medium heat, then add the garlic, chilli, anchovies and capers. Fry for 2 minutes, then add the tomatoes and toss well.
Reserving some of the cooking water, drain the bucatini and add to the sauce. Toss well over the heat until lovely and glossy, adding a splash of the cooking water to loosen, if needed. Flake in the tuna, then add some lemon juice and most of the rocket. Toss well to warm the tuna through and wilt the rocket, then season carefully with salt and more lemon juice. Serve with a drizzle of oil, and a scattering of the reserved rocket.