Place the blackcurrants, granulated sugar, lemon rind and juice in a saucepan and cook gently for 5 minutes.
Cool slightly, then puree in an electric blender or rub through a sieve.
Stir in the sherry.
Place the cream, caster sugar and half the blackcurrant puree in a bowl and whisk until the mixture forms soft peaks.
Spoon the remaining puree into the base of 2 glasses and top with the cream mixture.