Cook the bacon and set aside. Melt butter in a soup pot and add leeks, mushrooms and cauliflower, cook for 5 minutes; covered. Add chicken stock and water and bring to a boil then lower the heat and simmer for 10 minutes. Puree the soup in batches. Add the stilton to last batch of soup and puree until smooth. Heat through over low heat, if necessary. Top with crumbled bacon just before serving.