Put the gelatine and water in a small bowl and leave to become spongy. Stand the bowl in a pan of simmering water and leave it to become clear.
Meanwhile, add the blackberries to a pan. Add the lemon juice and cook over low heat until the juices come out, then increase the heat a little and cook until the fruit is soft.
Puree the blackberries in a food processor then sieve the puree to remove the seeds. Stir the dissolved gelatine into the hot puree.
Put the egg yolks and sugar in a clean food processor and process for a few seconds, then add the puree and mix well. Allow to cool.
Whisk the egg whites and add to the mousse along with the whipped cream.
Pour into individual glasses or a 2-pint glass dish and leave to set in the refrigerator.
Decorate with blackberries before serving.
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Notes
TIP: Don't cook the blackberries for too long, otherwise they will lose their bright colour.