Put the flour, salt, sugar and yeast in a large bowl and mix well. Heat the milk until lukewarm. Reserving a tablespoon of the yoghurt, add the rest to the milk, blending thoroughly. Melt the butter and add to the milk and yoghurt, mixing well.
Pour slowly over the flour and mix together, then knead until you have a springy dough. Leave the dough to rise in a warm place for about an hour, or until doubled in size. Divide the dough into 10 even-sized balls, cover and leave for about 15 minutes.
Preheat a moderate grill and put a large baking tray under it to heat for about 10 minutes.
Flatten the balls of dough into rough teardrop shapes and spread with the reserved yoghurt. Place on the hot baking tray and grill for about 2-3 minutes on each side, or until golden-brown. Keep a close eye on the flatbreads as they can burn very easily.