For the filling: Put the 200g sugar, 600g of the cherries and the lemon zest into a large pan over a medium heat. Put the cornflour and lemon juice into a small bowl and mix to form a smooth paste. As the cherries begin to release their juice, add this paste to the pan and cook the cherries gently, stirring, until the juice comes to the boil. Cook, stirring, for a few more minutes until thickened. Remove from the heat and add the reserved cherries. Set aside to cool while you make the pastry.
For the pastry: Mix the flour, salt and 2 tbsp sugar together in a bowl, then add the chilled butter and rub together until it resembles breadcrumbs. Slowly add the water until the pastry holds together when squeezed. Tip the dough onto the work surface and gently knead together. Divide into two balls, one a little larger than the other, then wrap them both in clingfilm and chill for 30 minutes.
Lightly dust the work surface with flour and roll out the larger piece of pastry until it is large enough to line a 23cm pie plate. Gently roll the pastry around your rolling pin and then drape it over the pie plate. Press the dough into the plate and then trim the excess leaving a 2.5cm overhang. Take the second piece of pastry and roll out as before. Then using a knife or pizza cutter, cut the pastry into strips 2.5cm thick. Put the pie plate and the strips of pastry into the fridge for 15 minutes to let the pastry rest.
To assemble: Preheat the oven to 200C/180C Fan/Gas Mark 6. Remove the pie plate and strips of pastry from the fridge and tip the filling into the prepared base. Brush the edge of the pie and the overhang with a little egg and water wash, and then lay half the pastry strips over the pie. Then lay the remaining half at a 90-degree angle from the first half, weaving them in and out. Then brush all strips with the egg and water wash.
To finish the pie, fold the pastry overhang so that it sits on the rim of the plate and covers the edges of the pastry strips. To crimp the pie, pinch together the thumb and forefinger of one hand against the edge of the pie crust and, using the index finger of the other, press the pastry together along the top edge to form a simple scallop. Scatter sugar over the lattice to create a caramelised golden finish. Bake for 40-45 minutes or until the pastry is golden and the filling is bubbling.