Combine the flour and the butter in a bowl and rub together until the mixture resembles fine breadcrumbs. Add the caster sugar and then a little cold water to bind to a dough. Wrap in cling film and pop in the fridge for 30 minutes.
In a saucepan, combine the cherries, demerara sugar, 2 dessertspoons port and nutmeg. Bring to a gentle simmer for 25 minutes until the cherries have softened.
Preheat the oven to 190 C / Gas 5. Grease a pie dish.
Remove the pastry from the fridge and roll out, adding a little more water if necessary, to about a 1 cm thickness. Roll enough to line the dish and make a lid.
Line the pie dish with the pastry then spoon in the warm filling. Slice the pastry lid into 2cm wide slices and keep the length of the pie dish. Intertwine the strips to form a lattice top. Brush with egg.
Bake in the preheated oven for 30 minutes or until the pastry is golden and the filling is piping hot. Remove from the oven.
To make the port wine crème, combine the double cream and 50ml port and whisk until light and fluffy.
Serve the pie with the port wine crème and decorate with mint leaves.