Warm the jam in a small pan over a low heat until loosened. Brush the fruitcake all over with half the jam. Lightly dust a work surface with icing sugar, then roll the marzipan out into a circle large enough to cover the top and side of the cake – it should be about ½cm thick. Lift the marzipan onto the cake using the rolling pin, then smooth the marzipan down the side and trim away any excess.
Add a few drops of blue food colouring to the fondant icing, then knead until the fondant is soft and pale blue (see tip, below).
Brush the marzipan-covered cake all over with the remaining jam. Roll the blue fondant out into a circle large enough to cover the top and side of the cake, then lift onto it using the rolling pin and smooth down the side. Trim away any excess fondant, and smooth out any marks using the palm of your hand. Set aside.
Mix the icing sugar with 1 tbsp water at a time until you have a thick, pipeable icing (you can use up to 3 tbsp water). Divide the icing between two bowls. Add a few drops of the green food colouring to one of the bowls (leaving one bowl plain), stir to combine, then transfer to a piping bag fitted with a medium round nozzle.
Pipe wiggly lines of the green icing all the way around the top and side of the cake a few times – this will be the ‘wire’ for the fairy lights. Use the tip of a cutlery knife or teaspoon to dot a small amount of the plain icing onto the backs of the sugared almonds, then arrange these along the piped green wire so the almonds resemble a string of fairy lights.