Run the tomatoes through a food mill to remove the seeds and skins and place in a large pot over medium heat. Stir in the stock, sugar, salt, bay leaf, basil and pepper. Bring to the boil, then reduce heat and simmer 30 minutes.
In a small saucepan over medium heat, melt butter. Whisk in flour all at once to form a roux, cook 1 minute. Whisk in milk, a little at a time, cooking and stirring constantly until thickened. Stir into tomato mixture and heat through.