Tip the blackberries (no need to defrost if frozen) into a saucepan with the icing sugar, lemon juice and 1 tablespoon of water. Over a low heat, bring to a simmer, stirring a few times (the berries should soften but not to stew). When they start looking juicy take off the heat.
Mash lightly to squeeze out all the juice and then pour into a nylon sieve set over a bowl. Push as much fruit through the sieve as possible, squeezing out every last drop of juice so you end up with a thin purée. Cool completely.
Meanwhile, whip the cream lightly in a separate bowl (don't whip it too much as the acidity in the fruit will also help it thicken). When the cream just holds a soft peak fold through the yogurt and half of the purée, until just combined. Ripple through the remaining purée and spoon into dishes. Top with some whole blackberries and serve.
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As seen in Real Food magazine