Slice the chocolate Swiss roll thickly and spread each slice with a little strawberry jam. Place the Swiss roll slices in the base of a trifle dish or glass bowl. Sprinkle over 4-5 tablespoons of reserved mango or peach juice and leave to stand for 10 minutes to let juice soak into the Swiss roll. Slice strawberries and dice mango or peaches and reserve some for decoration. Scatter evenly over the Swiss roll with half mango or peaches.
Break the chocolate into small pieces and place in a small heatproof bowl set over a saucepan of gently simmering water. Heat gently stirring occasionally until the chocolate has melted and is smooth and free from lumps.
Blend the custard powder, sugar, and milk to a smooth paste in a bowl, then pour into a heavy based saucepan. Place over a gentle heat and cook, stirring constantly, until the custard is thick and smooth. Add the melted chocolate and stir until smooth and blended. Remove from heat and leave to cool.
Spoon the custard mixture over the fruit and chill in the fridge for 1 hour. Whip the cream until soft peaks form and pile over the top of the set custard. Sprinkle over the toasted flaked almonds and decorate with the remaining strawberries and diced mango.