Destone the cherries and cut in half. Hull the strawberries, cutting any larger ones in half. Put the cherries, strawberries and blackberries in a saucepan with the sugar, cassis and 2 tbsp water. Simmer gently for 4-5 minutes, until the fruit is soft but retaining its shape. Stir through the raspberries. Drain the fruit through a sieve, retaining the juice.
Slice the loaf into 1cm slices and trim off the crusts. Line a 1.25 litre pudding bowl with clingfilm, then place a slice of brioche in the bottom of the bowl. Line the sides with the brioche, carefully overlapping them as you go so that the inside of the bowl is completely covered and there are no gaps.
Spoon the fruit into the bowl and pour over two thirds of the reserved juice. Use the remaining slices of brioche to cover the fruit and seal the pudding. Find a small plate that fits snuggly inside the bowl and place this on top of the pudding, weighting it with a couple of tins. Chill in the fridge for a minimum of 4 hours, ideally overnight.
To serve, remove the small plate and put your serving plate over the bowl. Turn it over and carefully remove the pudding basin and clingfilm. If there are gaps in the bread where the juice hasn’t saturated the bread, you can drizzle over any remaining juice.