1. Reserve a few whole blackcurrants for decoration.
2. Press the rest through a sieve into a measuring jug, then make up the puree to 150 ml (1/4 pint) with water.
3. Combine the blackcurrant puree, sugar and lemon juice in a mixing bowl.
4. Place 30 ml (2 tbsp) water in a small heatproof bowl and sprinkle the gelatine on to the liquid.
5. Stand the bowl over a saucepan of hot water and stir the gelatine until it has dissolved completely. Cool slightly.
6. Fold a little of the blackcurrant puree into the cooled gelatine, then whisk this mixture into the bowl of blackcurrant puree.
7. Leave in a cool place until the mixture starts to set.
8. In a deep bowl, whip the cream until it just holds its shape, then fold into the blackcurrant mixture with a metal spoon.
9. Whisk the egg whites in a clean, grease-free bowl, and fold in.
10. Make sure that the mixture is thoroughly and evenly blended but do not overmix.
11. Pour gently into a glass dish, a wetted 500 ml (17 fl oz) mould or individual glasses.
12. Refrigerate for 1 - 2 hours until set, then turn out if necessary and decorate with whipped cream and the reserved blackcurrants.