Cut the pumpkin into a small dice. Melt the margarine in a large pan, add the pumpkin and cook over a gentle heat for about 5 minutes.
Meanwhile peel and slice the potatoes. Add them to the pan and cook for a further few minutes. Then add the tomatoes, stock, salt and pepper. Bring to the boil and simmer very gently for one hour.
Allow to cool slightly before liquidising or putting through a sieve. Return the soup to the rinsed out pan, stir in the cream and re-season if necessary. Bring back to the boil. Sprinkle with a little chopped parsley before serving.