Place 1 cake layer in bottom of trifle bowl. Drizzle with a third of the sherry. Top evenly with 1 cup of custard. Place another layer of cake on top and moisten with more sherry. Top with blackberries, then 1 layer of cake, moistened with remaining sherry, and 1 cup custard. Top with all the preserves, then the third layer of cake, and remaining custard. Follow with cherries and the last layer of cake. Let refrigerate for 24 hours to set. Top with freshly whipped cream before serving.