In a bowl, mix together the crushed ginger biscuits and melted butter until well combined. Press this mixture into the bottom of each of six glass tumblers to form an even layer.
Bring the elderflower pressé and sugar to the boil in a pan. Continue to boil until the volume of liquid has reduced by half. Set aside to cool for 5-10 minutes.
In a separate bowl, beat the mascarpone, vanilla seeds, cooled elderflower pressé mixture and elderflower liqueur together until well combined. Fold in the whipped cream until well combined, then fold in the lychees.
Spoon the mixture into the tumblers.
In a bowl, mix the lime marmalade with the boiling water until it has dissolved. Set aside to cool slightly, then spread over the surface of each cheesecake.
Chill the cheesecakes in the fridge for two hours.
To serve, place each tumbler onto a small serving plate. Decorate each cheesecake with edible flowers, if using.