Put the sugar in a large saucepan with 5 tbsp water and 1 tbsp cassis (or 6 tbsp of water if not using cassis). Cook over a low heat until the sugar has dissolved, then turn up the heat and bubble for 6-7 minutes, until thick and syrupy. Add redcurrants and blackcurrants (if using) for 30-45 seconds, until they just start to burst, and remove from the heat. Leave to cool for 15 minutes, then stir in the remaining fruit. Leave for a further 20 minutes, to allow the fruit to release its juices.
Meanwhile, cut a small square from 1 slice of bread to fit the bottom of a 1·2 litre (2pt) pudding basin. Line the sides of the basin with 5 trimmed slices of bread in an overlapping layer. Press them in to fit snugly together, so there are no gaps for the fruit to escape.
Spoon the fruit into the bread-lined basin with a slotted spoon, packing it in tightly until level with the rim of the basin. Spoon over 2-3 tbsp of the juice and let it soak in. Reserve the remaining juice and any fruit. If necessary, trim any pieces of bread that stick up over the edge.
Fit the remaining slices of bread over the fruit, tearing smaller pieces, if needed, to patch up any gaps until the fruit is completely covered. Cover the top with a small plate or saucer and weight it down with some heavy tin cans. Sit the basin in a shallow dish to catch any juice and chill for 5 hours or overnight.
Meanwhile, pass the reserved juice and fruit through a sieve into a jug to make a sauce for the finished pudding. Cover and chill until needed.
Remove the weights and slide a palette knife around the edge of the pudding basin, to loosen the bread. Put a serving plate on top of the basin and then, holding the plate and basin in place, flip it upside down. The pudding should gently slide out of the basin. Brush any white patches of bread with the reserved sauce. Decorate with the extra summer fruit and serve with the sauce and some cream, if you like.
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