Peel the potatoes and roughly cut them into cubes. Cut off the broccoli stem - set the florets aside. Slice the celery and leek, chop then garlic.
Put the stock into a saucepan, add the broccoli stem, garlic, leek and celery. Add the bay leaf and bring to the boil.
Gently simmer the soup, under cover, for 15 minutes.
Add the broccoli florets, bring to the boil again, then cook for 5 more minutes.
Remove the soup from the heat, discard the bay leaf, then blitz with a stick blender until you get a smooth, silky soup.
Season with plenty of ground pepper before serving.