In this case, Warren J. said,
25 … any exercise of a case management power must accord with the overriding objective. I do not suggest that there is any tension between the overriding objective and the restriction of expert evidence to that which is reasonably required to resolve the proceedings. But I do suggest that what is reasonably required is informed by the overriding objective and that the court should not be over-zealous in excluding evidence in order to save time and cost.
61 Mr Tennet submits that expert evidence need only be helpful in resolving an issue justly. He refers to Barings Plc …What the judge actually said is that evidence can be excluded if the court comes to the conclusion that it would not be helpful in resolving an issue which needs to be decided where it is one of law or one on which the court can come to a fully informed decision without hearing such evidence but, whichever way one looks at it, evidence is admissible if it might be helpful.
But that is not, in my view, to say that it must be admitted even if helpful because it may be disproportionate in the light of the overriding objective to admit it. I accept, of course, that evidence can be helpful even if it is not determinative of any issue: see…Mitchell v News Group Newspapers Limited….
62 …Warby J’s observations must be read in the light of what he said later, being mindful of CPR 35.1 which also requires that the evidence must be “reasonably required to resolve the proceedings”. Thus at  he stated that he did not regard CPR 35.1 as imposing a test of absolute necessity. As he said,
“a judgment has to be made in the individual case, and it has to be made before the evidence is heard and evaluated. My conclusion is that evidence which it was credibly said could conclusively determine the single most important issue in the case meets the criterion in the rule.”
63 This, it seems to me, is saying something very different from the proposition that, because expert evidence may prove of assistance, it should be admitted.
A judgment needs to be made in every case and, in making that judgment, it is relevant to consider whether, on the one hand, the evidence is necessary (in the sense that a decision cannot be made without it) or whether it is of very marginal relevance with the court being well able to decide the issue without it, in which case a balance has to be struck and the proportionality of its admission assessed.
In striking that balance, the court should, in my judgment, be prepared to take into account disparate factors including the value of the claim, the effect of a judgment either way on the parties, who is to pay for the commissioning of the evidence on each side and the delay, if any, which the production of such evidence would entail (particularly delay which might result in the vacating of a trial date).
64 Let me get one point out of the way. CPR 35.1 refers to “the proceedings”. In the present case, a large number of pleaded issues arise. It may, in the end, be unnecessary to resolve some, perhaps many, of those issues and I rather suspect that that will be the case. But it is only at the trial that it will become apparent what issues actually need to be decided. I say “trial” but that needs to be qualified because the court can, in the exercise of its case management powers, restrict the issues for determination at a CMC: see CPR 3.1(2)(k) . But no one has sought, thus far, to exclude any of the pleaded issues from consideration.
Accordingly, it cannot be said, at present, that it will not be necessary to decide any particular pleaded issue in order to resolve the proceedings. It must follow that, if expert evidence is reasonably required to resolve a pleaded issue, it will also be reasonably required to resolve the proceedings. However, unless the evidence is necessary in order to resolve an issue, whether it should be admitted needs to be assessed in the context of the resolution of the proceedings as a whole.
There would be nothing inconsistent in accepting that particular evidence ought to be admitted in resolving an issue within the proceedings if that issue stood alone but deciding, in the context of the proceedings as a whole, that such evidence was not reasonably required in resolving the proceedings (unless that evidence was necessary to resolve the issue).
65 Thus in Mitchell , the expert evidence was not necessary but it was, or might have turned out to be, helpful. Because the issue to which it went was central to the case and because the evidence might be conclusive, it was admitted. But if in another case a similar issue were to arise which, instead of being central, was merely peripheral, the court might take the view that the expert evidence was not reasonably required to resolve the proceedings. The balance could come down in favour of refusing to admit that evidence.
68 …it is necessary to look at the pleaded issues and, unless and until a particular issue is excluded from consideration under CPR 3.1(2)(k) , the court must ask itself the following important questions:
(a) The first question is whether, looking at each issue, it is necessary for there to be expert evidence before that issue can be resolved. If it is necessary, rather than merely helpful, it seems to me that it must be admitted.
(b) If the evidence is not necessary, the second question is whether it would be of assistance to the court in resolving that issue. If it would be of assistance, but not necessary, then the court would be able to determine the issue without it (just as in Mitchell the court would have been able to resolve even the central issue without the expert evidence).
(c) Since, under the scenario in (b) above, the court will be able to resolve the issue without the evidence, the third question is whether, in the context of the proceedings as a whole, expert evidence on that issue is reasonably required to resolve the proceedings. In that case, the sort of questions I have identified in paragraph 63 above will fall to be taken into account. In addition, in the present case, there is the complication that a particular piece of expert evidence may go to more than one pleaded issue, or evidence necessary for one issue may need only slight expansion to cover another issue where it would be of assistance but not necessary.
69 Further, although CPR 35.1 does not refer to issues, but only to proceedings, if evidence is not reasonably required for resolving any particular issue, it is difficult to see how it could ever be reasonably required for resolving the proceedings. I therefore see a test directed at issues as a filter. That, at least, is an approach which can usefully be adopted.
This case has been edited for ease of reading. See also the other cases posted under this tag (including Barings and Mitchell).