Put the oil, onions, celery, carrots, potatoes and bay leaves in a big casserole dish, or two saucepans. Fry gently until the onions are softened – about 10-15 mins. Fill the kettle and boil it.
Stir in the tomato purée, sugar, vinegar, chopped tomatoes and passata, then crumble in the stock cubes. Add 1 litre boiling water and bring to a simmer. Cover and simmer for 15 mins until the potato is tender, then remove the bay leaves. Purée with a stick blender (or ladle into a blender in batches) until very smooth. Season to taste and add a pinch more sugar if it needs it. The soup can now be cooled and chilled for up to 2 days, or frozen for up to 3 months.
To serve, reheat the soup, stirring in the milk – try not to let it boil. Serve in small bowls for the children with cheesy sausage rolls then later in bowls for the adults as Hot Bloody Mary soup (see 'Goes well with' recipes, below).