Crush the biscuits and put them in a bowl. Add the melted butter and mix well.
Place the mixture in the bottom of a few jam jars or in one large tin, then press down firmly to form an even base.
Soak the gelatine leaves in a bowl of cold water for about 5 minutes, until soft.
In a small saucepan, heat the elderflower cordial and lime juice until it simmers. Drain the gelatin and add it to the pan. Mix until the gelatin has completely dissolved.
Mix the cream cheese, mascarpone and sugar until smooth. An electric mixer makes this job easier. Add the gelatin mixture and mix well.
Put the cream filling over the base of the biscuits and put it in the fridge to cool and set.
To make the elderflower jelly
Soak 4 more gelatine leaves in a bowl of cold water.
In a small saucepan, add the juice of the 4 limes, 50g of caster sugar, 4 tablespoons of elderflower cordial and 125ml of water. Bring the mixture to a simmer.
Add the softened and drained gelatine leaves, and mix until completely dissolved.
Pour the jelly over the cheesecake and put it in the fridge overnight to set.
Decorate the top of the cheesecake with elderflowers before serving.