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 with cheesy sausage rolls.