Halve and stone the greengages. Place in a non corrosive saucepan with 3 tbsps water and 115 g sugar. Cover and simmer, stirring occasionally, over a moderate heat for 10 minutes or until the fruit is soft.
Puree until smooth then strain and set aside until cool. Cover and chill.
Whisk the cream until it forms soft peaks. Sweeten the yoghurt with the remaining sugar and fold into the cream. Partially stir this into the cold greengage puree so that you have green and white swirls. Divide between 6 glasses or bowls then cover and chill until ready to serve.