Heat olive oil in a saucepan over medium heat. Stir in the onion and garlic; cook until tender. Mix in courgette and coat in the olive oil. Pour in passata and water. Season with oregano, basil and crushed chillies. Dissolve sugar in the sauce. Reduce heat to low and simmer 1 hour, stirring occasionally.
During the last 10 minutes of the simmer time, bring a large pot of lightly salted water to the boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
In a large bowl, mix sauce and cooked pasta shells. Top with cheese to serve.