For the jelly, heat the blackcurrants, blackberries, sugar, 50ml/2fl oz water and crème de cassis in a pan until boiling. Reduce the heat until simmering, then simmer for 1-2 minutes without stirring.
Strain the mixture through a sieve lined with muslin, catching the strained juice in a clean pan. Tip the cooked fruit back into the original saucepan.
Heat the strained juice over a medium heat, then stir in the gelatine until melted. Set aside to cool.
For the blackcurrant mousse, add the frozen mixed berries to the pan of cooked berries, pour over 50ml/2fl oz water and bring the mixture to the boil. Cook for 2-3 minutes, mashing the fruit in the pan as it cooks. Stir in 2 tablespoons of the cassis.
Transfer the mixture to a food processor and blend to a fine purée. Strain the purée through a fine sieve into a clean saucepan, pressing the fruit through the sieve using the back of a spoon.
Return the strained mixture to the heat and stir in the gelatine until dissolved. Set aside until completely cooled, then chill in the fridge, covered, until needed.
Heat the sugar and 50ml/2fl oz water in a pan until the sugar has dissolved and the mixture is boiling. Continue to boil the syrup until its temperature reaches 118C/235F (this is called the soft ball stage). Use a sugar thermometer to test the temperature.
Whisk the egg whites until soft peaks form when the whisk is removed. Gradually pour the hot sugar syrup onto the egg whites in a thin but steady stream, whisking continuously, until all of the syrup has been incorporated (take care to make sure the syrup doesn't hit the beaters as it may splash you). Continue to whisk for a further 1-2 minutes, to cool the mixture slightly.
In a separate bowl, whisk the double cream until soft peaks form when the whisk is removed.
Whisk the fruit purée into the egg white mixture, then whisk in the double cream.
Use a 20-22cm/8-8½in metal chefs’ ring to cut out the centre of the flan case. Cut the sponge in half horizontally to create two thinner sponge discs. Reserve or freeze one for use in another recipe.
Place the chefs’ ring onto a serving plate or cake stand and place one sponge disc inside it. Drizzle over the remaining cassis, then pour the blackcurrant mousse mixture inside, almost but not quite to the top. Chill in the fridge for at least 1 hour, or until the mousse has set enough to support a layer of jelly.
Pour the jelly on top of the cake to form a thin layer. Return to the fridge for another 2 hours until set firm.
Decorate the top of the cake with the figs, raspberries and sprigs of mint. When ready to serve, run a hot towel or blow torch briefly around the edge of the chefs’ ring to loosen it, then lift away.