Combine the flour, sugar, salt and yeast in a large mixing bowl, then work the olive oil in. Add the warm water steadily until you can form a fairly sticky dough. Add a little more water if it is too stiff, then turn out onto a lightly floured surface and knead for 8-10 minutes until smooth and elastic. Place in a large, clean mixing bowl and cover with clingfilm.
Leave to rise in a warm place (an airing cupboard for example) for 2 hours or until doubled in size.
Meanwhile, melt the butter in a saucepan over a medium heat and sweat the leek with a little salt for 6-8 minutes, stirring occasionally, until soft and translucent. Remove from the heat and set to one side.
Grease a large baking tray. Turn the dough out onto a lightly floured surface, knock it back and knead again for 3-4 minutes until smooth and elastic. Divide into 4 pieces and shape into shallow ovals. Divide the leek mixture between the dough, placing in the centre of each.
Sprinkle the grated cheese on top of the leek, then fold two of the doughs over and around the filling, sealing the edges well to enclose the filling totally. Fold the other two pieces of dough over but don't seal these. Cover loosely with an oiled piece of clingfilm and leave to rise again for 10-15 minutes.
Preheat the oven to 210°C.
Make light incisions in the top of the two unsealed pieces of dough. Bake for 25-35 minutes until golden and risen. Remove and allow to cool on a wire rack for 10 minutes before serving.