Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Line 2 x 12-hole muffin trays with paper cupcake cases.
In a mixing bowl, stir the cake mix, oil, water and eggs gently together, then whisk (by hand or electric mixer) for 2-3 minutes until smooth and creamy. Divide the mixture evenly between the cupcake cases. Bake both trays together in the oven for 18-22 minutes. Remove from the oven and carefully transfer the cupcakes to a wire rack to cool completely; about 30 minutes.
Put the icing in a large bowl and use the green food colouring paste or gel to colour the icing bright green. Reserve 1 tablespoon of icing. Use a small palette knife to spread the remaining icing on the top of the cupcakes.
Using the tubes on black writing icing, pipe 4 concentric circles on each iced cupcake. Gently draw the tip of a cocktail stick through the circles from the centre outwards at intervals to create a spider’s web pattern on each cupcake.
Use a rolling pin to flatten 24 of the pastilles into ovals. Cut each into 8 thin strips and place the strips on top of each cupcake to form the spider’s legs. Top each with a whole pastille for the spider’s body using the reserved icing to secure in place. Using the black writing icing, pipe tiny eyes on the spiders. Serve immediately or keep in a cool place for up to 2 days.