If using frozen blackberries, remove from the freezer to defrost and drain in a sieve, discard any liquid.
In a food processor, blitz the plain flour, butter and sugar until the mixture resembles fine bread crumbs.
Add two egg yolks and 4 tablespoons of chilled water. Mixture should form into a smooth dough, add additional water if required.
Flatten dough into a rough disk shape, wrap in gladwrap and refrigerate for 30 minutes.
Once refrigerated, portion the mixture into thirds. Roll out two thirds of the dough to a 0.5cm thickness. Lightly grease 4x 12cm fluted pie tins. Line the tins with the rolled pastry. Trim excess pastry with a knife, ensuring to leave a 1cm overhang of pastry. Roll out the remaining third to a 0.5cm thickness. Slice strips of dough for the lattice top, at least 40 strips are required, approximately 14cm by 0.5cm.
In a bowl mix together all of the blackberry filling ingredients, set aside
Place a layer of apple slices in the bottom of the pie, top with the blackberry filling.
Create and top the pie with the lattice pattern with 10 strips for each pie (5 strips vertical, 5 strips horizontal). This can be done directly on the pie or on the bench and transferred to the pie. Gently press the edges of the lattice into overhanging dough, trim any excess lattice strips.
Cover the pies with gladwrap and refrigerate for 30 minutes.
Set up your barbecue for indirect cooking. Please refer to your handbook for further instructions. Preheat your barbecue for indirect baking.
Using a pastry brush, brush the tops of the pies with the lightly beaten egg. Sprinkle the pies with demerara sugar.
Once the barbecue has preheated, make adjustments to the burner control(s) if required. Place the pies on and cook for 35 minutes, checking at 20 minutes. If tops of the pies are becoming too dark, place a sheet of alfoil on top of the pies to prevent from burning.
Remove the apple and blackberry pies from the barbecue once cooked, allow to stand for a couple hours for the filling to set.