What is the position of a non-party opposing the enforcement of a subpoena?

The position of the non-party opposing the enforcement of the subpoena was well-put by Denning, M.R. in Senior and others v. Holdsworth, [1975] 2 AllE.R. 1009 at p. 1013: If the person summoned takes objection to it, he can wait till the trial and take his objection there. In the old days, if he did this, he had to do it himself and was not allowed counsel to do it for him… Nowadays, of course, he can do it by counsel. The better course is, however, for him to apply before the trial to set aside the summons … The summons will be set aside if the witness has no material documents or if it is oppressive or there is any other sufficient reason to set it aside.