Melt the butter in a large pan, add the leeks and mushrooms and cook for 5 minutes or until soft.
Add the milk, stock cubes, water, mushroom ketchup and ground bay leaves.
Bring to the boil and simmer for 20 minutes.
Blend the cornflour with the sherry. Stir into the soup and cook for 2 minutes.
Garnish with chopped chives to serve.
If mushroom ketchup is unavailable, the soup is also good with Worcestershire sauce instead which has a very similar flavour, although the anchovy content makes it unsuitable for vegetarians.