Tip the sugar into a large frying pan with 2 tablespoons of water. Heat slowly, stirring occasionally, until the sugar has dissolved. Bring to the boil, then lower the heat to medium, add the plums and poach gently for 1-2 minutes, or until softened but still holding their shape. Remove the pan from the heat and lift out a few plum slices to reserve for decoration. Stir in the blackberries and set aside to cool.
Spread the jelly or jam onto 4 of the cake slices. Sandwich together with the rest of the slices, then cut into chunky cubes. Pile these in the bottom of a trifle dish. Stir the cassis (if using) into the cold syrup, then pour enough of the syrup over the cake to moisten it (if your plums are very juicy and you have some syrup leftover it can be served on the side). Spoon the fruit over the cake, reserving a few blackberries for decoration.
Spread the custard over the fruit, then top with big spoonfuls of the cream (if the cream is not thick enough to spoon on straight from the pot, tip it into a bowl and beat briefly with a fork). Decorate with the reserved fruit, and serve.
The poaching time for the plums will depend on how ripe they are, but they should only take a few minutes, as you don’t want them to get too soft and lose their shape.