Peel the apples and cut into quarters then cut into thin slices. Place in a large bowl and drizzle with the lemon juice and vanilla.
Mix the sugar, cinnamon and flour in a small bowl. Add the flour mixture to the apples. Stir everything well. Add the sultanas.
Lay out the puff pastry. Place the filling down the centre. If there is too much don't use it all.
Fold the top of pastry over the filling and brush with the egg-milk mixture. Fold the bottom of pastry over it and press firmly. Seal the ends with the egg-milk mixture.
Turn the strudel over so that the nice side is facing up. Lay on a buttered baking tray and brush it with the remaining egg-milk mixture. Pierce the top a few times with a knife so the steam can escape.
Bake the strudel in the middle of the oven for 20 minutes. Reduce the temperature to 190 degrees C and bake for another 20 minutes until golden brown until.
Leave to cool for at least 20 minutes before cutting. Dust with icing sugar.