"What above all needs to be emphasised is that there is no balancing exercise to be performed under rule 13. The weighing of loss of confidentiality on the one hand against litigious disadvantage on the other is, obviously, difficult enough at the best of times: these are wholly disparate interests not readily matched against one another. Such a task is, of course, necessary if and when a prima facie claim to public interest immunity is made out. It is not, however, desirable to introduce this difficulty in some diluted form into the present type of rule 13 proceedings; that rather is wholly unnecessary and inappropriate: see Lord Wilberforce in Science Research Council v Nasse . ..."