Mix the flour, sugar, baking powder and nigella seeds in a bowl. Add in the melted butter and yogurt, then add 180 to 200ml water slowly until it becomes a dough. Then kneed for 5 mins until elastic, cover and rest for 1hr. Divide into 10 to 12 pieces and return to the bowl. Set the Retigo to 240oC and place in the pizza tray to heat up. Roll out the balls of dough, then when ready, place into the oven. after a few minuets, open the door and check the naan's. They may need turning over at this stage.