Preheat the oven to 275 degrees F (140 degrees C). Line a large baking sheet with parchment paper.
Put the sugar and lemon juice in a large bowl. Add one egg white and whisk for a minute - the mixture will look hard, but don't worry. Add another egg white and whisk for a few minutes before adding the remaining whites. Then whisk for 4 to 5 minutes until the meringue is stiff and shiny.
Dollop the mixture in a circle on the parchment paper about 8in (20cm) round, making the sides slightly higher than the center. Bake in the bottom of the oven for 1 to 1 1/2 hours, or until the meringue is firm and crisp on the outside but still soft and "pillowy" inside. If you can, wedge the oven door open a tiny bit with a damp tea towel; this allows the moisture to escape and dries out your meringue more quickly. You don't want too much color - it may begin to turn a very, very pale beige, but that is it. Once cooked, turn off the oven and leave the meringue inside until completely cool.
Meanwhile, put all the fruit filling ingredients except the blackberries in a medium saucepan set over medium heat. You can throw the vanilla seeds and the bean into the mix. Bring the liquid to just boiling point, then turn down to a poach. (A poach is when there is only one or two bubbles breaking the surface of the liquid.) Cook for about 15 minutes, until the fruit is soft but still has a bit of bite. Add the blackberries and remove the pan from the heat, then let the fruit cool in the liquid.
To make the cream filling, put the cream, sugar, and vanilla into a large bowl and whip. The cream is whipped enough when it just starts to hold its shape in the bowl and does not run when the bowl is tipped. It is best to very slightly underwhip, as the cream will thicken when left to stand.
Drain the fruit through a sifter or colander, reserving the liquid if you want, to have as a hot toddy.
To assemble the pavlova, place the meringue on a serving plate. Dollop the cream mixture all over it, leaving a 1 1/2 (4cm) border all the way around. Pile the drained fruit all over the cream and serve.