Preheat oven to 180 degrees C. Grease and flour an 20cm square cake tin.
In large bowl, cream together margarine and 1/2 cup sugar. Beat in eggs, one at a time. Combine flour, baking powder and salt. Fold dry ingredients into butter mixture. Pour into prepared tin.
Bake 25 minutes, or until cake springs back when lightly touched in centre. Cool in tin for five minutes, then remove from tin and cool completely on wire rack. Cut into narrow pieces about 4x10cm. Set aside.
In large bowl, beat cream with electric mixer until soft peaks form. Beat in 1/4 cup sugar and vanilla and continue to beat until stiff peaks form. Set aside.
Prepare vanilla pudding according to package directions. Set aside.
To Assemble Trifle: Brush each piece of cake with raspberry jam. Use half the cake pieces to line the bottom of a trifle bowl or other glass serving dish. Sprinkle half of the sherry over cake. Layer half the peaches, strawberries and blueberries on top. Cover with half the pudding and a third of the whipped cream. Repeat layers with remaining cake, sherry, fruit and pudding. Top with remaining whipped cream. Chill in refrigerator at least 30 minutes before serving.