Put the sugar and 300 ml (1/2 pint) water in a heavy-based pan and heat gently until the sugar has dissolved. Boil for 10 minutes until syrupy, then remove from the heat and set aside to cool.

Put the blackcurrants in a pan with the lemon juice and heat gently for about 10 minutes until softened. Allow to cool lightly, then puree in a blender. Press the pureed. blackcurrants through a sieve into a bowl to remove seeds and skin.

Sprinkle the gelatine over the 2 tablespoons water in a heatproof bowl and leave to soak for 5 minutes until spongy. Stand the bowl in a pan of gently simmering water and heat gently for 1 - 2 minutes stirring occasionally until the gelatine has dissolved. Stir the gelatine into the cooled sugar syrup.

Stir the sugar syrup into the blackcurrant puree and mix well. Turn into a rigid container and freeze, uncovered, for about 3 hours until the mixture is firm around the edges.

Remove the blackcurrant mixture from the freezer and break up with a fork. Whisk the egg white until it stands in stiff peaks, then fold into the blackcurrant mixture. Cover and freeze overnight, until solid.

To serve: stand at room temperature for about 30 minutes until the sorbet is soft enough to scoop into individual glasses.