For the base: Rub the butter into the flour in a medium bowl until it resembles breadcrumbs. Stir in enough water to form a soft, sticky dough. Cover with plastic wrap and leave to rest for 30 minutes.
Roll out the pastry on a floured surface large enough to fit a oiled 23cm flan dish. Line dish with pastry.
Preheat oven to 350 degrees F.
For the filling: Mix all the filling ingredients together in a large bowl. Pour filling into the pastry case. Bake in oven for 1 hour or until just set.