Mixed Berry Pie (topped with jammy biscuits)

Name: Mixed Berry Pie (topped with jammy biscuits)
Cuisine: American
Ingredients: 230g (just over 8 oz) Ready made pastry (for pie crust); 500g (17½ oz) fruit (fresh or frozen); 2 tbsp water; 1 tbsp cornflour / cornstarch; 4 tbsp sugar (any); 1 tsp vanilla sugar / vanilla extract; 1 packet jam filled biscuits