Once the marzipan is dry on your cake, brush it all over with water just to dampen the surface. Cut off 750g of the icing, re-wrap the rest, then dust the worktop with icing sugar. Knead the icing until pliable and smooth, then roll out and lay over the cake in the same way as you did with the marzipan (see 'Goes well with' recipe). Leave to dry completely overnight.
For the stars, cut the remaining icing in half. Knead a small spot of icing colour (you can always add more later) into one half until evenly pale blue. Cut in half, then work a little more colour into one piece to make it darker.
Piece by piece, roll out the white and blue icings a little thicker than a £1 coin. Stamp out about 40 stars, re-rolling any trimmings, then push the piping nozzle or straw through the middles to make holes. Lift onto a rack, then dry in a cool, dry place for 24 hrs, or longer if you can. Once dry, knot an 80cm length of ribbon loosely at one end, then thread with stars. Untie the knot, then tie the ribbon in a bow, making sure that the stars are slanting in the same direction.
Once the cake is dry, pick up the wreath of stars carefully, then lay it on top of the cake. Fix the remaining ribbon around the bottom of the cake using a little tape or glue.