In a saucepan, heat together milk, sugar, margarine and salt until warm.
Combine 250g flour and the yeast in a large mixing bowl. Stir in warm milk mixture. Add 3 eggs; beat with an electric mixer at low speed for 1/2 min, scraping sides of bowl constantly. Beat for 3 minutes at high speed. Stir in as much of remaining flour as you can. Turn out onto a lightly floured surface. Knead in remaining flour to make a stiff dough and continue kneading for 6 to 8 minutes. Place dough in a lightly greased bowl and turn once to grease surface. Cover and let rise in a warm place until doubled.
Meanwhile, pour boiling water over poppy seeds; let stand 30 minutes. Drain thoroughly and place in a liquidiser. Grind. In a small bowl, blend together ground poppy seeds, chopped walnuts, honey and lemon zest. Beat 1 egg white until stiff and fold into poppy seed mixture.
Punch down dough and divide in half. Cover and let rest for 10 minutes. On a lightly floured surface, roll half of the dough into 24x20cm rectangle. Spread with half the poppy seed mixture. Roll up like a swiss roll, starting at the narrow edge. Seal the edge. Place seam side down, in a greased 20x10cm loaf tin. Repeat with the other half of the dough. Cover. Let rise for 30 to 45 minutes or until doubled.
Bake in a preheated 180 C / Gas 4 oven for 35 to 40 minutes.