Mix the berries and cherry jam together, and spoon into your trifle bowl. Scatter over the Madeira cake and push down a little into the berries.
Put the custard, mascarpone and 3 tbsp sugar in a bowl together and beat until smooth, then dollop all over the trifle. Put the double cream, remaining 2 tbsp sugar and sherry in a bowl and beat until soft peaks form. Cling film both the trifle bowl and cream and chill until ready to serve. You can do this up to 24 hours in advance.
To serve, spoon the sherry cream all over the trifle, then scatter with your favourite decoration.