Heat the oil in a large pan, add the leeks and cook on a low-medium heat for 5 mins. Pour in the bouillon, tip in the beans, cover and simmer for 10 mins. Stir in the garlic and spinach, cover the pan and cook for 5 mins more until the spinach has wilted but still retains its fresh green colour. Add the milk and plenty of pepper, and blitz with a stick blender until smooth. Ladle into bowls and chill the remainder.