Peel and stone the avocados. Cut them up quite small and place in a food processor or blender. Scrape all the flesh from the skins and add this to the machine. Blend the mixture to a smooth paste, then remove from the machine and rub through a fine sieve. Add both the cream and the yoghurt to the avocado purée together with the lemon juice. Mix in well and season to taste with salt, pepper and a little sugar.