Make up jelly following the instructions. This should make about 2 cups of jelly.
Use 1 1/4 cups of the liquid jelly mixture to fill the bottoms of 6 small dessert dishes or wine glasses.
Put the 6 glasses and remaining mixture into the fridge.
When the remaining mixture is getting close to setting (but still able to be poured (this may take 1-2 hours), combine the mixture with cream and beat on high for 4 minutes.
The jelly in the cups should almost be set by this stage. Carefully pour the jelly and cream mixture (flummery) to add another layer to each of the glasses.
Allow to set.
Crush sponge fingers in a tray and then pour over port, sherry or muscat. Substitute juice if you'd prefer a kids or non-alcoholic version of the trifle.
Add sponge fingers as the next layer and then pour a layer of custard in each glass.
Add chopped berries as another layer, reserving 6 berries for decorating.
Whip thickened cream and custard powder until the cream forms stiff peaks.
Pipe cream onto top of layers and decorate each trifle with a berry and mint leaf.