Place 350g strawberries and caster sugar in a heatproof bowl over a saucepan of simmering water and cook gently, stirring occasionally, for 1 hour.
Soak gelatine in a small bowl of cold water for 5 minutes.
Strain berry mixture, discarding solids. Squeeze excess water from gelatine, add to warm strawberry syrup and stir until dissolved. Stir in Champagne, skimming any foam from the top. Cool slightly, then refrigerate for 15020 minutes until almost set.
Dip savoiardi pieces into the dessert wine, then divide among four 1 cup dessert glasses. Divide the remaining 100g berries among glasses, then pour over the strawberry jelly mixture. Refrigerate for 203 hours until jelly is set.
Spoon over the creme patissiere, then refrigerate for a further 30 minutes or until set.
Meanwhile, whisk cream and icing sugar to soft peaks. Serve trifles topped with cream and crumbled amaretti.