Sift the flour, sugar, baking powder and salt. Warm the milk and yoghurt, and add the yeast, butter and egg. Mix to combine thoroughly. Make a depression in the centre of the flour and pour in the mixture a little at a time until all is absorbed. Knead well for 15 minutes till smooth and springy, adding a little more flour if the dough is sticky. Cover and leave to rise until double in bulk (approximately three four hours, unless the weather is hot, in which case the dough might rise in half the time). Divide into eight portions and roll into balls with well floured hands. Cover and set aside for another 15 minutes. Flatten each ball into a thick pancake with the help of a little flour by tossing from one palm of the hand to the other. Brush tops with melted butter or egg yolk and sprinkle with poppy seeds. Bake in a very hot oven (4500 F. Gas Mark 8) for about 10 minutes. The nan should puff up and brown. An alternative method is to cook on a hot griddle and when one side is done, place raw side up under a grill.