Place the leeks into a pan with the butter, cover and cook slowly over a low heat for around twenty minutes. Add the potatoes and bay leaves and cook for a further five minutes, stirring occasionally. Pour in the water and bring to the boil. Turn the heat down and leave to simmer for twenty minutes. Remove the bay leaves and add crumble in the cheese, then whizz the soup into a creamy puree in a food blender. Season with black pepper and stir in the yoghurt. Serve with a little crumbled cheese stirred in at the last minute. Source: Our Collection - Farmison