Make a sugar syrup by heating the caster sugar with 100ml/3½fl oz water until the sugar has completely dissolved. Set aside to cool slightly.
Put the cherries in a large pan with 50ml/2fl oz of the sugar syrup and half the cherry brandy. Cook for 5-10 minutes, then set aside to cool.
Whisk together half the cream and all the custard.
Soak the cake slices in the remaining brandy and 50ml/2fl oz sugar syrup.
In a large serving bowl, create the layers. First add the cherries, then the soaked cake and top with the cream and custard mixture.
Put the remaining whipped cream in a piping bag and pipe swirls onto the top of the trifle. Cover with the grated chocolate.
To decorate, melt the sugar into a non-stick pan set over a medium heat. Keep heating until a dark-brown caramel forms (CAUTION: Do not leave unattended), then immediately dunk the pan in cold water to prevent the caramel burning.
Carefully dip the cherries into the caramel and place on a silicone mat or non-stick baking parchment to cool completely. When cool decorate the trifle with them.