Preheat the oven to 180ºC, fan 160ºC, gas 4. Grease the silicone muffin tray well with a little of the sunflower oil.
In a large bowl, put the oil, flour, sugar, squash, orange zest, walnuts, mixed spice, baking powder, golden syrup and eggs. Beat together with a handheld electric whisk until combined.
Spoon the mixture into the silicone tray (each hole should be ¾ full) and bake for 25 minutes. Cool completely.
Make the icing: put the water, lemon juice and icing sugar into a bowl. Mix well, until completely smooth. It should be thick enough to coat the back of a spoon - if needed, add more icing sugar to thicken, or a little warm water to thin.
Drizzle the white icing over the skulls and into the eye, nose and mouth indentations of the pumpkins. Decorate with the currants, sprinkles and writing icing - kids will love helping with this step!