Taken from page 808 where  Slade L.J. in the judgment of the court had said,

Expert evidence from suitably qualified professional persons is, in our judgment, admissible to show what competent architects in the position of [the Defendant] could reasonably have been expected to know and do in their position at the relevant time. Indeed, in our judgment, there could be no question of the court condemning them for professional negligence on account of their failure to appreciate points (3) and (4) and to take the suitable consequential action, unless there were appropriate expert evidence to support the allegation that their conduct fell below the standard which might reasonably be expected of an ordinarily competent architect…

The expert evidence before the judge consisted of evidence from three engineers and one architect, Mr Foster. The questions put to the engineers and answered by them included questions relating to the nature and extent of the professional duties owed by [the Defendant] to [Another]. However, we think that little reliance can be placed on their answers to these questions, which relate to a profession other than their own. The only directly relevant evidence in this context was that of Mr [X], who was called on behalf of [the Defendant].

Investors was followed in Sansom which is also cited under this tag. Edited for ease of reading.