Heat the oil in a large pan, and cook the leeks and garlic gently until softened. Stir in the ginger and cumin, and cook for a minute. Add the pumpkin and stock, season and simmer on a low heat for 30 minutes until the pumpkin is tender. Process in a blender. Warm gently without boiling when ready to eat, garnishing with coriander leaves and a swirl of yogurt.