For the cake: Preheat the oven to 180°C (160° fan) | 350F | gas 4. Grease and line a 23 cm | 9" springform cake tin with greaseproof paper on the base.
Beat together the margarine and sugar in a large mixing bowl until smooth and creamy, about 2 minutes.
Beat together the eggs and vanilla extract in a separate bowl. Beat the egg mixture into the creamed margarine by the tablespoon and then fold in the flour, xanthan gum, baking powder, and salt until you have a smooth, even batter.
Divide the batter into two bowls. Add red food colouring to one, and green to the other, stirring well until each batter is uniformly coloured.
Spoon half of the batters into the prepared cake tin and sprinkle the raspberries and chocolate candies on top. Cover with the remaining batter and then swirl the two batters together using a cocktail stirrer or a wooden toothpick; the pattern can be as random as you like.
Bake for 35-45 minutes until risen and springy to the touch on top; a cake tester should come out clean when inserted into its centre. Remove to a wire rack to cool.
For the icing: Once cool, turn out the sponge onto a cake stand. Dust a work surface with a little icing sugar and then roll out the white fondant icing on it to 0.5 cm | 0.25" thickness.
Cut out an approximate 40 cm | 16" circle of the icing. Level the top of the sponge with a serrated bread knife before brushing with the warmed apricot jam.
Drape the circle of fondant over the cake, positioning it so that the sides drape evenly around the edges. Use a pastry scraper or your hands to help smooth the icing over the edges down to the base of the sponge. Cut away the excess icing with a sharp paring knife and set aside for later.
Roll out the black fondant in the same manner as the white. Cut out bat and cat shapes using cutters or freehand using a paring knife. Set aside.
Colour off cuts of the reserved white icing with green and yellow food colouring, kneading well until uniformly coloured. Fashion both colours into eyes for the animals on the cake.
To decorate: Melt the chocolate with the butter and Golden Syrup in a heatproof bowl set over a half-filled saucepan of simmering water.
Remove from the heat when ready and give it a quick stir. Leave to cool and thicken for 5 minutes before spooning into a piping bag fitted with a nozzle with a very thin round tip.
Pipe a spider's web onto the top of the cake, with strands coming down the sides. Use some of the warmed jam to secure the black fondant animals to the top and sides of the cake, along with their colourful eyes. Sprinkle over the two kinds of sugar.
To serve: Cut the cake with a damp, sharp knife for clean cuts. Serve with more chocolate candies on the side.