Cut out a piece of card very slightly smaller than the width of the top of the plastic bowl. Cover with cling film.
Bring the water and sugar to the boil in a wide pan. Add the strawberries and blackcurrants and simmer for 1-2 minutes.
Remove from the heat, stir in the remaining berries and allow to stand for 4 minutes. Drain the mixture, putting the berries on a tray and the berry juice in a bowl.
From one slice of bread cut a disc slightly larger than the base of your pudding bowl. Dip the disc in the berry juice for 20 seconds, then place in the base of the pudding bowl.
Set one slice of bread aside for the lid and cut the remaining slices in half. Soak them in the juice and use to line the sides of the pudding bowl, overlapping slightly.
Spoon the berries into the dish, pushing down firmly. Press them down so that the berries are level with the top of dish. You should have some berries left over. Save them for the sauce.
Soak the remaining slice of bread in the juice and place it on top of the dish, pushing down slightly. Fold any overhanging pieces of bread from the sides onto the top.
Place the cardboard disc on the top of the summer pudding and rest something flat and heavy on top to press the bread lid down on the berries.
Place the pudding on a plate to catch any juice and leave overnight.
Put the remaining mixed berries in a saucepan with the caster sugar and add some berry juice left over from earlier to give the sauce a smooth creamy consistency.
Bring to the boil and simmer for 2 minutes.
Remove from the heat and blend using a hand mixer.
Sieve into a jug, discarding the seeds, and allow to cool.
Remove the pudding from the bowl and turn it out carefully onto a plate.
Spoon some of the berry sauce over the top. Serve with clotted cream and a jug with the remaining berry sauce.
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