Preheat the oven to 180°C, fan 160°C, gas 4.
Meanwhile, heat the oil in a heavy-based frying pan, add the onion, celery, if using, pepper and garlic, and fry gently for 10 minutes until really soft. Add the tomatoes, tomato purée and sugar and simmer gently, uncovered, for 15 minutes, until thickened.
Meanwhile, cook the pasta according to pack instructions and drain. Put the pasta in an ovenproof dish. Sprinkle the tuna over the top. Cover with the tomato sauce and carefully mix together.
Sprinkle the cheese and herbs over the top. Crumble the toast into breadcrumbs and sprinkle over too. Bake in the oven for 15 minutes, until the top is golden and the pasta is heated through.