Cook the pasta following pack instructions, reserving a mugful of the pasta water and draining it 2 mins before the stated time. Heat 2 tbsp of the oil in a deep, non-stick pan over a low-medium heat, then tip in the aubergine. Stir well to coat in the oil, frying until softened and deeply golden all over, around 10-15 mins. Remove to a bowl and pour the remaining oil into the pan. Once hot, tip in the onion with a pinch of salt and fry for 5-7 mins until just softened before stirring in the peppers and courgettes. Cook for a further 15-17 mins until lightly browned then add the garlic. Cook for a couple of minutes until fragrant. Stir in the tomato purée and allow it to caramelise for 5 mins. Pour in the cooked aubergine, tomatoes, basil stems and half of the basil leaves. Bring to a gentle simmer. Leave to bubble away for 20-25 mins, then season to taste.