Heat the oil in a large frying pan and fry the pancetta for 3-4 minutes, until starting to crisp. Add the onion, carrot and celery and continue to sweat over a low heat for 8-10 minutes, until the vegetables are soft. Add the garlic, rosemary and tomato purée and cook for 2-3 minutes.
Add the mince to the pan and turn the heat up. Use your spoon to break up the mince and fry for 3-4 minutes, until browned. Add the Worcestershire sauce, ale and chopped tomatoes and bring to the boil. Reduce the heat and simmer for 20-25 minutes, until the sauce has reduced down to a thick, sticky sauce. Season well with black pepper.
Meanwhile, cook the spaghetti according to the instructions on the packet. Drain and return to the pan. Stir a couple of large spoons of the sauce through the pasta, then divide between four shallow bowls. Top with the remaining bolognese sauce. Garnish with grated parmesan.