1) Preheat oven to 180C/Gas Mark 4.
2) In a large bowl, combine peaches, tapioca, sugar, and salt. Gently mix in berries and vinegar. Taste for balance and add more vinegar, if desired. Spoon mixture into a 20 by 20-cm baking dish.
3) In a large mixing bowl, combine flour, salt, 3 tablespoons brown sugar, and baking powder. Work in the butter using your fingers or a pastry cutter until the mixture forms large pea sized lumps. This can also be done with the paddle attachment of a stand mixer, or with brief pulses in a food processor. Slowly mix in 175ml double cream until just combined.
4) Gather dough together into a ball and knead gently on a lightly floured board until it holds together. Roll out to 1-cm thickness. Using a 7-cm round or star cutter, cut dough into shapes. Place them decoratively on the fruit mixture. Brush pastry with 1 tablespoon of cream and then sprinkle with remaining 1 tablespoon brown sugar. Bake until pastry is golden brown and the fruit is bubbly, about 40 minutes. Put cobbler under grill for a few seconds to brown pastry, if necessary.
5) Beat remaining 240ml of cream in a medium bowl with a whisk or electric mixer on high. Gradually beat in 55g granulated sugar. Beat until it holds soft peaks, and then add the vanilla. Serve with cobbler.