Heat the olive oil in a large lidded pan or casserole and add the onion, carrot, celery, courgette and garlic with a large pinch of salt. Stir well, put on the lid and cook gently for 20 minutes, stirring every now and again, until the veg is very soft. Pour 500ml of boiling water into a jug and whisk in the Marmite and tomato purée. Pour into the pan along with the crispy turkey mince, scraping the bottom of the tray to get any crispy bits. Season and simmer gently for 30 minutes or until thickened.