Make strawberry jelly following pack instructions. Set aside to cool. Pour into a 30 x 20cm tin. Cover. Refrigerate for 4 hours or until set.
Cut sponge cake into 3cm cubes. Halve or slice strawberries and put in a bowl with blueberries and raspberries. Put cream in the small bowl of an electric mixer and beat, using whisk attachment, until soft peaks form.
Put pieces of sponge cake in the base of a round trifle dish. Using a spoon, drizzle liqueur over sponge cake, soaking each cube. Roughly cut set jelly into squares and arrange over top of sponge cake.
Scatter ½ of the fruit over jelly. Spoon ½ of the cream, then all the custard over fruit. Spoon over remaining cream, then fruit. Dust with icing sugar before serving.
Allow 4 hours to settle before serving.
