Heat the oil in a large pan and add the butter, leeks and potatoes. Cook over a medium heat for 10 minutes. Meanwhile, in a large jug, make up 1¼ litres of vegetable stock using the stock cube.
Add the stock to the pan and bring to the boil. Add the chopped broccoli, then cover and simmer for 15 minutes.
Meanwhile, slice the bread into 8 rounds and place under a hot grill until golden. Toast the other side of the bread, top with the cheddar and grill until bubbling.
Crumble the stilton into the soup then blend in a food processor until smooth. Pour into 4 bowls, season and top each with 2 cheesy croutons.