“Another factor which seems to me to be relevant is the nature of the undertaking itself. Two features may be important. First, it is for the respondent to show that loss has been suffered, and that this has resulted from the grant of the injunction. Secondly, the Court retains the power, and duty, to decide whether, in all the circumstances, the respondent should be compensated for that loss. This must of course be done in a principled way. But by the same token, it must mean that in reaching a decision the Court should take into account the general rule against awarding compensation for loss caused by administrative action undertaken on behalf of the public, and in the name of the public interest.”