Combine prunes and port in covered container; let stand overnight.
Add mincemeat and walnuts to prune mixture; and set aside. Roll half the pastry out on floured board and use to line 22 cm (9-inch) pie plate; trim and flute edges. Spoon prune-mincemeat filling into pastry shell. Roll out remaining pastry; brush with egg; sprinkle with sugar. Bake in 220C (425F) oven 10 minutes. Reduce heat to 190C(375F); continue baking 20 to 25 minutes until crust is golden and filling is bubbly. Cool on rack.
Serve wedges with whipped cream, sweetened to taste.