To make the pastry: In a food processor, pulse the flour, salt, baking powder and butter until the mixture resembles coarse crumbs. Continue to pulse with just enough water to get the dough to come together. Press into a flattened disc and wrap in cling film. Chill for 30 minutes.
Roll the pastry out on a lightly floured surface and use to line a deep pie dish. Flute the edges with thumb and finger. Prick the base of the pastry case with a fork.
To make the filling: Combine the pumpkin, sugar, eggs, spices, milk and sultanas together in a bowl. Pour into pastry case and bake in 220 C / Gas 6 oven for 10 minutes. Reduce oven temperature to 180 C / Gas 4 and continue to bake until a knife comes out clean when inserted into the centre.