To make the base, mix together the crushed biscuits and melted butter, and press into a lightly greased 23cm spring form tin. Put in the fridge to set for 1 hour.
Grease with oil and line a 23cm springform tin with clingfilm. Whisk together the condensed milk, soft cheese, lemon juice and orange zest and juice.
In a separate bowl, whisk the cream until soft peaks form, then fold into the cream cheese mixture with the raspberries, blackberries and strawberries, reserving a few for decorating. Put into the cake at put in the fridge to set for 2-3 hours
When set, remove the cheesecake from the tin and clingfilm. Decorate with the reserved raspberries and strawberries. Dust with the icing sugar before serving.