Preheat the oven to 140°C, fan 120°C, Gas Mark 1. Line two large baking sheets with nonstick baking paper.
Whisk the egg whites in a large bowl until stiff, moist looking peaks. To check they are ready, turn the bowl upside down, the egg whites will only slide if not whisked enough. Gradually whisk in the sugar a teaspoonful at a time until it has all been added then whisk for a minute or two more until very thick and glossy. Mix the cornflour with the vinegar then fold into the meringue.
Spoon into 24 small mounds on the paper lined trays, leaving a little space between them. Spread into rough shaped circles about 5 cm (2 inches) in diameter then make a small dip in the centre with the back of a teaspoon. Bake for 25-30 minutes until the meringues are very pale biscuit colour and can be easily lifted off the paper. If they stick, cook for another 10 minutes then test again. Leave to cool on paper.
Mix the jam and lemon juice together. Divide the fruit between three bowls then stir a third of the jam into each. Whip the cream until it forms soft swirls, fold in the yogurt.
Take pavlovas off the paper and arrange on a tray or plate, spoon cream on top then add raspberries to 8 mini pavlovas, blueberries to 8 more and strawberries to last batch. Serve within 1 hour of decorating.
Tip: the pavlovas can be made the day before, when cold wrap the trays loosely with foil and keep in a cool place. Top with cream and fruit when needed.