For the jelly, place the gelatine leaves in a bowl of cold water to soften. Squeeze out any excess water from the leaves. Place the sugar syrup in a saucepan over a very low heat and gradually add the gelatine, stirring until the leaves melt. Do not allow the mixture to boil. Add the sherry and stir to mix. Strain the jelly mixture through a sieve into a shallow baking tray. Chill in the fridge until firm. Once set, cut the jelly into 4cm/1.5in squares.
For the custard, heat the cream and vanilla seeds in a saucepan until just boiling. Remove from the heat and beat in the yolks and sugar. Return the pan to a low heat and stir until the custard thickens. Remove from the heat, strain through a sieve and set aside.
For the sponge, preheat the oven to 170C/190C Fan/Gas 3 and line a Swiss roll tin with baking paper. Beat the eggs, vanilla extract and sugar together in a large bowl and fold in the flour. Spoon the mixture into the tin and level the surface evenly with a spatula. Cook for 20–25 minutes, or until slightly risen and firm. Cover with a tea towel as it cools to keep the sponge soft. Once cool, heat the cherry jam and lemon juice gently in a small saucepan. Spread the jam over the sponge, leaving a slight gap around the edges, and carefully roll it up. Cut into rounds.
For the cream, beat the mascarpone and cream together in a bowl until soft peaks form.
For the chocolate sauce, place the chocolate and sugar in a glass bowl over a saucepan of simmering water, taking care that the bowl does not touch the water. Slowly stir until the sugar is dissolved and the chocolate has melted.
To assemble, add the cherries and blackberries to a large, decorative glass serving bowl, reserving a few for the top. Add the jelly squares and decorate the sides of the bowl with the Swiss roll rounds. Layer in the custard, cream, chocolate sauce and cherry syrup. Decorate with the chocolate shavings, pecans and almonds and top with the remaining cherries and blackberries.