1. Melt the butter in a small pan and stir in the crushed biscuits. Lightly press into the base of a 23cm/9in deep loose-bottomed flan tin. Chill whilst preparing the filling.
2. To prepare the filling: place the lime juice into a large bowl, add the cream and condensed milk. Whisk for 1-2 minutes. Add the lime zest and lightly stir. Pour mixture into the prepared biscuit base. Place on a tray and chill for 1-2 hours.