Preheat the oven to 100°C.
Heat greengages with 2 tablespoons of water and 120g sugar until broken down and syrupy.
Whisk egg whites to soft peaks, then whisk in the remaining 150g caster sugar to stiff peaks.
Sift in icing sugar and whisk until the mixture is very stiff and glossy.
Line a baking tray with paper using small dots of meringue to secure the corners.
Fold half the cooled fruit into the whites for a ripple effect and spoon eight dollops onto the tray.
Bake for 2 hours then leave to cool completely in the oven with the door propped open.
Whisk double cream with vanilla seeds until soft peaks form.
Sandwich two meringues together with cream and fruit on a plate and serve immediately.