Firstly, get all the fruit ready. Hull the strawberries and cut them in halves or quarters depending on how big they are. Strip the blackcurrants and redcurrants from their stalks in one fell swoop by running a fork down the length of each stem – keep both the currants separate from the other fruits.
Tip the sugar into a wide, not too deep, saucepan. Measure in 3 tbsp water and the cassis. Put the pan on a low heat and cook, stirring often, until you can no longer hear the crunch of sugar grains on the bottom of the pan. When the sugar is dissolved, turn up the heat to medium-high and let the mixture bubble away for about 8 minutes. It will go quite syrupy and you want to catch it just before it starts to change colour or caramelise.
Now tip the blackcurrants and redcurrants into the hot syrup, it will feel quite sticky at first, then bring everything back up to a lively simmer and let it bubble again for no more than a minute, just to lightly burst and soften the currants without losing their shape. Take the pan off the heat and leave until it is barely warm.
Gently stir in the strawberries and raspberries – a large metal spoon is best so they don’t break up – and let the fruity mixture sit for about half an hour so the juices all mix in.
Cut 4-5 slices from the loaf, about 5mm thick, and trim off the crusts. Cut a little square (about 4cm) from one slice and put it in the bottom of a 1.2 litre pudding basin. Using a big slotted spoon, put a layer of fruit (about 3 spoonfuls) over the bread. Next lay a slice of bread in the centre over the fruit trimming to fit and fill any gaps with trimmings of bread so the fruit is covered. Continue layering with more fruit, more bread, then a final layer of fruit so it comes to within a hair’s breadth of the top of the basin. Spoon over a few spoonfuls of juice – not too much or it will ooze out when weighted down. (You should have about 4 spoonfuls of fruit and juice left for making a sauce.) Cover the fruit with a final layer of bread, press down to compact everything, then cover with cling film. Lay a saucer on top and weight down with heavy cans or weights. Stand the basin on a plate in case any juices spill out, then leave in the fridge overnight, or for a minimum of five hours. Press the leftover fruits and juice through a metal sieve to make a sauce, keep chilled. (You can freeze the pudding and the sauce at this stage for up to a month.)
To turn out, go round the edge of the pudding with a round-bladed knife to release it, then invert it on to a plate. Cut into slices with a serrated knife and serve with a drizzle of the fruit sauce and cream.