The injunction in question didn't specifically forbid the Guardian from reporting on the parliamentary question. The injunction forbade the Guardian from reporting anything about the Minton report, and it forbade the Guardian from reporting that it had been injuncted.
The Minton report is an internal report, commissioned by Trafigura, into the dumping of waste in Ivory Coast. The report was leaked to Wikileaks.org.
Paul Farrelly's question mentioned the Minton report and the injunction. And so Carter-Ruck sent a letter to the Guardian claiming that reporting on Farrelly's question would constitute a breach of the earlier injunction and thus a contempt of court.
The judge, when he/she granted the injunction, didn't know anything about the parliamentary question as it hadn't been asked yet. Nor even mooted.
Paul Farrelly was effectively trying to use parliamentary privilege to draw attention to this disgraceful and ludicrously broad injunction. The Guardian hoped that his question would allow it to effectively reveal to its readers that it had been injuncted from publishing details about the Minton report (available to us all on wikileaks.org).
Carter-ruck, in sending the letter to the Guardian, have shot themselves in the foot. But presumably, next time, they will be careful to get an injunction forbidding the Guardian from reporting on letters received from Carter-Ruck.
As this article points out, the twitterati have not solved the problem. They have merely alerted Carter-Ruck to a loophole that I am sure they will be careful to plug next time.