Melt chocolate and butter in a bowl over saucepan of hot water. Stir in crushed biscuits and press into a flan dish. Chill.
Put marshmallows and milk into a bowl over a pan of hot water and stir occasionally until melted. Allow to cool slightly. Stir in grated rind and juice from the limes.
Whisk cream until fairly stiff and fold into marshmallow mixture. Pour onto biscuit base. Chill until set. Decorate.