Place the mince in a large non-stick saucepan with the onion, celery, carrots and garlic. Dry-fry over a medium heat, stirring from time to time with a wooden spoon for 8-10 minutes, or until the beef is no longer pink and the vegetables are beginning to brown.
Add the mushrooms and fry with the mince and vegetables for another 2-3 minutes. Sprinkle over the flour and stir well, then add the wine, tomatoes and tomato purée, along with 300ml cold water.
Crumble the stock cube over the top then add the caster sugar and herbs. Season with a few twists of freshly ground black pepper, stir well and bring to a simmer.
When the liquid is bubbling, reduce the heat and simmer very gently for 30-40 minutes, uncovered, stirring occasionally.
Bring a large pan of salted water to the boil. As soon as the water is boiling, add the spaghetti and push it down with a wooden spoon to encourage the strands to separate. It's important to use lots of water so the spaghetti can move freely without sticking together.
Cook for 10-12 minutes, or according to the packet instructions.
While the spaghetti is cooking, increase the heat under the sauce and simmer for another 10 minutes or until the liquid is reduced and the sauce is rich and thick. Stir regularly and season to taste with salt and freshly ground black pepper.
Drain the spaghetti in a colander and divide between warm bowls. Spoon the bolognese sauce on top, sprinkle with freshly grated parmesan cheese and serve immediately.