Tip the flour into a large mixing bowl, add the chilled, diced butter, poppy seeds, turmeric and cayenne, and season with salt and freshly ground black pepper. Rub the butter into the flour using your fingers.
When the butter is almost incorporated (but still with some small flecks visible) and the mixture resembles fine sand, add the ice-cold water and vinegar.
Mix to combine, first using a palette knife, adding a drop more water if needed, and then using your hands, to bring the dough together into a ball. Do not overwork or knead the dough.
Flatten into a disc, wrap the dough in baking parchment and chill for a couple of hours or until firm.
Meanwhile, prepare the filling. Peel the potatoes, cut into dice and cook in boiling salted water for about 10 minutes until tender. Drain and leave to cool.
Slice the onions and leek, tip into a frying pan, add the olive oil and cook over a low- medium heat until softened but not coloured. Add the garlic and cook for another minute before adding the potatoes.
Season the mixture well with salt and freshly ground black pepper and mix gently to combine. Remove from the heat and leave to cool.
When the potato mixture is cold, add the grated cheese and snipped chives. Lightly dust the work surface with flour and divide the dough into 6 even portions. Roll the dough out, then cut into discs 16-18cm in diameter, using a plate as a guide.
Lay the discs on the work surface and spoon the chilli jam into the middle of each. Divide the filling between the pastries, spooning it into a neat and compact mound in the middle of the pastry.
Brush the edges of the pastry with water and fold over to encase the filling in a half moon shape. Brush the edge again with water and decoratively crimp the edge to seal.
Arrange the pasties on a parchment-lined baking tray and chill in the fridge for 20 minutes.
Meanwhile, preheat the oven to 180°C/Fan 160°C/Gas 4. Brush the pasties with milk, sprinkle with poppy seeds and make a small steam hole in the top of each using a skewer.
Bake on the middle shelf of the preheated oven for 30 minutes until the pastry is crisp and golden brown.