Heat the oil in a deep, wide frying pan or casserole dish over a medium-high heat. Tip in the meatballs and cook for 5 mins, turning until browned all over. Add the onion and garlic, and fry for 8 more mins until softened.
Add the ketchup, chopped tomatoes, chopped basil and 400ml water to the pan and bring to the boil. Add the spaghetti and cook for 10-12 mins, stirring occasionally. When the pasta is cooked and the sauce has reduced, season and sprinkle with the basil leaves to serve.