Add the garlic, bay leaves and chillies to the softened onions, stir around the pan for 1-2 mins, then pour in the passata, Worcestershire sauce, remaining vinegar and sugar. Season the sauce, give a good stir, then put the brisket on top. Wrap the tin tightly in a few sheets of foil, or cover with a lid, and bake for 6 hrs, turning once or twice during cooking, until really tender.