Start by making the pastry, if making your own. Do this by adding the flour and butter to a large bowl and rub together with your fingertips until roughly combined. You do not want to take it as far as breadcrumbs, there should still be bits of butter visible in places. Add the water and make into a ball. Chill 30 mins.
Meanwhile, make the filling: peel and chop the potatoes into small chunks (5cm dice). Boil until cooked then drain. Finely slice the leeks and add to a large frying pan with some butter. Cook on a low/medium flame until softened.
Once the leeks are cooked, add in the cooked potatoes, some dried rosemary and salt and pepper. Turn the heat up to medium and add a splash of wine (optional), before adding the cheese and cream cheese. Stir together until combined into a cheesy, creamy mixture. Set aside to cool.
Remove the pastry from the fridge, roll in one direction to form a long rectangle. Fold the top edge down to just below the middle. Then fold the bottom edge over this, to form a square with 3 layers (book end). Rotate this block 90 Degrees and repeat. Wrap and chill for another 20 minutes.
Preheat the oven to 220C/200C Fan/Gas 7. Line two/three large baking trays.
Roll the pastry into a large rectangle, about 18×4 inches. Spread the potato filling across the long edge, in a cylinder/log shape and press together to keep tight. Leave about an inch of pastry to one side, and more room on the other side.
Take the edge of the pastry and roll it over the filling, keeping it as tight as possible. Keep rolling the pastry until you reach the end and have a log of pastry with the filling all inside.
Cut the pastry log into about 10 sections. Place on baking sheets, at least 2 inches apart. Optional: brush a little milk on the top and add some more dried rosemary. Bake for approx 15-20 minutes, until golden brown. Let cool on a wire rack (although they are fabulous eaten when warm).