Remove the strings from the celery and dice.
Peel the potatoes and cut into cubes.
Peel the garlic and chop finely.
Warm the oil in a pan and fry the garlic gently for a moment. Add the celery and potatoes and fry over a medium heat. When the celery and potatoes are warmed through, add the leeks and fry over a low heat making sure that it doesn't burn.
Add the water and crushed stock cube and bring to the boil over a strong heat. Remove any froth that appears. Lower the heat and cover, and simmer for 15 minutes.
Blend to a puree and season to taste.
When serving, garnish with parsley and fresh cream to taste.