Put the double cream in a heavy saucepan, add the mace and cinnamon and heat gently until just simmering. Stir in 1 tablespoon of the sugar and all but 2 tablespoons of the rosewater and finally the rennet. Turn the mixture into a wide, shallow bowl. Leave in a cool place for 2 hours or longer.
Just before serving pour over the single cream mixed with the 2 tablespoons of rosewater and sprinkle with the remaining sugar. Stand the bowl on a larger flat dish and surround with the rose petals. Serve a few petals with each helping so that the diner may smell roses as he eats.