Add a splash of olive oil to a large saucepan or pot. Add the leek and garlic and cook until soft - about 10 minutes. Add the potato and chicken stock and bring to the boil. Lower heat and simmer until the potato is soft. Add the celery and bring to the boil and cook for about 5 minutes or until the celery is just soft. Add the parsley and double crew; stir and remove from the heat.