Put sugar and 1 1/4 cups water in a heavy-based saucepan and stir to dissolve. Bring to boil and boil for 5 minutes. Remove from heat; stir in lemon juice and let cool. Stir mango purée into syrup and pour into a shallow metal container. Chill the mixture for 30 minutes the cover and freeze to the slushy stage. Turn semi-frozen mixture into a bowl and beat well. Return the mixture to the container, cover and freeze until just becoming slushy again. Turn out into the bowl and beat well. In a separate bowl, whip egg white until stiff but not dry. Fold cream and then the egg white into the mango mixture, spoon back into the container, cover and freeze until firm. About 30 minutes before serving, transfer the sherbet to the refrigerator.