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For the compote, hull strawberries and halve or slice if large. Put currants and blackberries (if using) into a pan with the sugar and 3 tbsp water and bring to the boil, stirring. Simmer for 5 mins until softened, add remaining fruits and cook for a couple more mins. Remove from the heat, add cassis, if using, then cool.
Line the base of a 1kg loaf tin with a long strip of double thickness baking parchment, allowing some overhang at the narrow ends to make lifting easier. Slice the cake into long thin strips to allow for 3 layers. Use offcuts for 'patching'.
Cover the base of the tin with one layer of cake, trimming offcuts to fit snugly. Spoon in half the fruits with some juice on top. Fit another layer of cake on top, then the rest of the fruits and finally a top layer of cake. Reserve leftover juice to pour over the cake when it is turned out. Press lightly down to squash the fruits into the cake, cover with cling film and put something heavy on top to weight it down. Chill overnight.
Transfer to a plate, using the paper to help loosen and lift it out. Spoon enough of the reserved juice over to moisten (any left over can be used as a drink with fizzy water). Serve with a bought or homemade coulis or cream.