Mix together the crushed biscuits and melted butter in a bowl, then press into the base and up the side of a 20 cm (8 inch) fluted loose-bottomed flan tin. Chill until firm.
Beat together the mascarpone, lemon rind and juice, double cream and icing sugar in a large bowl until thick, then spoon over the biscuit base. Chill for at least 30 minutes until firm.
Decorate with blueberries, raspberries and lemon rind curls, when ready to serve.