Place 75g (3oz) of sugar in a small pan with 100ml (3½fl oz) water. Heat gently and stir until dissolved. Add the strawberries and raspberries to the pan.
Mash a couple of raspberries, take off the heat and allow to cool for 15 minutes. Strain into a shallow dish, reserving the fruit.
Dip the sponge fingers one by one into the syrup and use to line the base and sides of a 1 litre (1¾pt) pudding basin, cutting the sponges to fit.
In another pan, soak the gelatine in 4tbsp cold water for 5 minutes until soft, then warm gently until melted.
Place the yogurt in a bowl with the vanilla extract and the remaining sugar, then beat in melted gelatine. Place half the mix into the basin, add the reserved fruit, cover with rest of mix and then with any leftover sponges. Drizzle any remaining syrup around the edge and chill for 4 hours until set.
To serve, invert onto a plate, remove the basin and top with some extra berries.
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