Preheat the oven to 100C/80C Fan/Gas ¼.
In a saucepan, heat the greengages with 2 tablespoons water and 120g/4¼oz of the caster sugar over a medium heat for 15-20 minutes, stirring occasionally, until the greengages have broken down and are coated in a sticky syrup. Remove the pan from the heat and set aside to cool completely.
Whisk the egg whites in a clean bowl using an electric whisk until soft peaks form when the whisk is removed. Add the remaining 150g/5½oz caster sugar and whisk again until stiff peaks form when the whisk is removed.
Sift in the icing sugar and continue to whisk the egg whites until the mixture is really stiff, glossy and resembles shaving foam.
Line a baking tray with baking paper or silicone, dabbing a dot of meringue mixture in each corner to stick the paper to the tray.
Carefully fold half of the cooled greengage mixture into the beaten egg whites until you have created a ripple effect. Spoon eight large dollops of meringue mixture onto the baking tray, leaving a gap between each.
Bake the meringues in the oven for 2 hours, then turn the oven off and open the door slightly (propping it open with a wooden spoon is a good trick). Leave the meringues to cool completely in the oven.
Meanwhile, whisk the double cream with the vanilla seeds until soft peaks form when the whisk is removed.
To serve, spread a circle of the reserved greengage mixture in the centre of each plate. Place one meringue on top and spoon over the whipped cream. Top with a second meringue, then spoon over more greengages. Serve immediately.