Preheat oven to 350 degrees F (175 degrees C).

Grease and flour an 8x8 inch cake pan.

In a large bowl, cream together margarine and 1/2 cup sugar until light and fluffy.

Beat in eggs, one at a time, until well combined.

In a separate bowl, combine flour, baking powder, and salt.

Gradually fold the dry ingredients into the butter mixture until just combined.

Pour the batter into the prepared cake pan.

Bake for 25 minutes, or until a toothpick inserted into the center comes out clean.

Cool in the pan for five minutes, then remove and cool completely on a wire rack.

Cut the cooled cake into narrow pieces about 1 1/2 inches by 4 inches.

In a large bowl, beat heavy cream with an electric mixer until soft peaks form.

Gradually beat in 1/4 cup sugar and vanilla extract until stiff peaks form.

Prepare vanilla pudding according to package directions.

To assemble the trifle, brush each piece of cake with raspberry jam.

Use half the cake pieces to line the bottom of a trifle bowl or glass serving dish.

Sprinkle half of the sherry over the cake.

Layer half of the peaches, strawberries, and blueberries on top of the cake.

Cover the fruit with half of the prepared pudding.

Top the pudding with a third of the whipped cream.

Repeat the layers with the remaining cake, sherry, fruit, and pudding.

Finish by topping the trifle with the remaining whipped cream.

Chill in the refrigerator for at least 30 minutes before serving.