Preheat oven to 350°F (180°C), Gas mark 4. Line 2 bun tins with 24 paper cases. Cream the butter and caster sugar together until light and fluffy, either with a hand-held electric mixer or in a freestanding mixer.
Gradually add the eggs, mixing well after each addition and adding a teaspoon of the flour if the mixture starts to curdle. Fold in the flour until thoroughly combined.
Spoon the mixture into the paper cases, filling each one halfway full. Bake for 15-20 minutes until well risen and golden brown. A skewer should come out clean when pushed into the middle of a cake.
Remove the fairy cakes from the bun tins and leave to cool on a wire rack.
Meanwhile, make the icing. Sift the icing sugar into a bowl and add enough of the water to make a smooth paste. Spoon the icing over the cooled fairy cakes and while the icing is still wet decorate with assorted sweets and sprinkles.
To serve, arrange the decorated fairy cakes on a cake stand and watch the faces light up as you bring it to the table!