Butter a pie dish, then roll out the pastry to fit the dish. It should be about the thickness of a pound coin.
Place the pastry into the dish and trim so it's still overhanging. It will likely shrink when baked so don't trim off too much. Crimp the edges and place in the freezer for at least an hour.
Preheat oven to 220°C (200°C fan assisted) and place a baking tray in the oven.
In a sauce pan, combine the sugar, flour, salt and water. Bring to the boil and stir constantly until it's thick and smooth.
Add the cranberries, juice and cinnamon. Cook for a further 5 minutes until it's bubbling and the cranberries have started to pop.
Remove from the heat and add stir through the butter.
Spoon the filling into the prepared pie dish, use any left over pastry to make a lid, this could be a lattice, stars or strips. I made snowflakes to keep it Christmassy.
Wash the pastry with milk or egg and place onto the baking tray in the pre-heated oven.
Bake for 40 minutes, you may need to cover with foil after about 25-30 if it's browning too quickly.
Remove when the filling has set. Allow to cool completely before serving.