You will need a 1.1 litre (2 pint) glass dish or six small dishes.
Tip the blackberries, lemon juice and 75g of the caster sugar into a saucepan. Stir, cover the pan and simmer for five to eight minutes until soft. Pass the blackberries through a sieve back into the pan, then discard the seeds. Reheat until piping hot.
Put the gelatine leaves into a bowl of cold water and leave for five minutes. Squeeze the water from the gelatine leaves and add them to the hot blackberry juice. Stir until dissolved. Set the mixture aside until it is cold and has thickened slightly.
Whip the cream to soft peaks. In a separate, clean bowl, whisk the egg whites, adding the remaining 100g of caster sugar a teaspoon at a time. Keep whisking until all the sugar has been incorporated and the whites are stiff and look like a cloud (as for a meringue). Take care not to over whisk or it will be tricky to incorporate the egg whites into the blackberries.
Add two large tablespoons of the whipped cream to the blackberry and gelatine mixture and stir in gently. Carefully fold in the rest of the cream and the egg whites until the mixture is smooth and light, with no white bits visible. Pour into the dish or dishes and place in the fridge for about six hours, or ideally overnight, to chill and set.
Decorate with a few blackberries, dust with icing sugar and serve with some whipped cream.