Mix the flour, salt and yeast in a large bowl. Make a well in the centre, add the oil and water, then mix well. If the dough seems a little stiff, add 1-2 tbsp water, mix again, then put on a lightly floured work surface and knead well. Place the dough in a lightly oiled bowl. Leave to rise for 1 hour until doubled in size.
Remove from the bowl, knock back the dough, then spread out with your hands onto a large, oiled baking tray 20cm x 30cm. Brush with olive oil and make deep indentations with your fingers. Layer the potatoes and rosemary leaves over the top and drizzle with a little more oil. Leave to prove for 1 hour until doubled in size.
Heat oven to 230C/fan 210C/gas 8. Sprinkle the dough with a little rock salt, if you like. Bake the bread for 30 mins until it is golden, the potatoes are tender and it sounds hollow when tapped underneath. Drizzle the loaf with more olive oil, if you like and, when cooled, serve on the tray.