Made the Chocolate Sheet Cake yesterday, but the icing didn't set properly and was very liquid.
Nigella's Chocolate Sheet Cake is a chocolate sponge cake that has a chocolate icing poured over it while the cake is still warm. The icing is partly absorbed by the warm cake and the rest of the icing sets to a fudgy consistency. The icing should not set firm, but neither should it remain liquid.
If the icing is still runny then there are a few possible reasons. In very warm conditions the icing may not cool enough to set quite fully. You could try putting the cake in a cold place or the fridge for a couple of hours to help to cool the icing and set it and then store the cake in a cool place. Also if you have used too much liquid then the icing will be too runny. Measure the milk carefully and use butter, as a "spreadable" butter or soft margarine may make the icing softer. Finally the cake must be warm when the icing is poured over, otherwise the cake will not absorb it as well. Once the cake cools the crust becomes harder and the icing will sit on top rather than sinking into the cake and may be more runny.