What is the test for setting aside a subpoena in a medical malpractice case?

Henry L.J. agreed with Staughton L.J. and stated (at p. 5): Subject to the above I am of opinion that Seyfang v. Searle, [1973] 1 QB 148, [1973] 1 All ER 290 remains good authority for the proposition that when it comes to setting aside a subpoena a distinction, at any rate in the exercise of the court’s discretion, should be made between expert witnesses and witnesses of fact. That case, dealing as it was with the position of what I might describe as the conventional independent expert who gives evidence as part of his profession, is not exhaustive in the principles it lays down. Nor should it be construed as a statute.