Crush the digestive biscuits into fine crumbs and mix with the melted butter and demerara sugar.
Press the mixture firmly into a lined 20cm springform tin and chill for 30 minutes.
Beat the cream cheese, soft cheese, condensed milk, icing sugar, lemon zest, lime zest, lemon juice, lime juice, and vanilla extract until smooth.
Whip the double cream to soft peaks and gently fold it into the cheesecake mixture.
Spoon the filling onto the chilled biscuit base and smooth the top.
Cover and refrigerate for at least 6 hours or overnight until fully set.
Remove from the tin and decorate with lemon slices, lime slices, lemon zest curls, and a light dusting of icing sugar before serving.