“The extent of the legal duty in any given situation must, I think, be a question of law for the court. Clearly, if there is some practice in a particular profession, some accepted standard of conduct which is laid down by a professional institute or sanctioned by common usage, evidence of that can and ought to be received. But evidence which really amounts to no more than an expression of opinion by a particular practitioner of what he thinks that he would have done had he been placed, hypothetically and without the benefit of hindsight, in the position of the defendants, is of little assistance to the court; whilst evidence of the witnesses’ view of what, as a matter of law, the solicitor’s duty was in the particular circumstances of the case is, I should have thought, inadmissible, for that is the very question which it is the court’s function to decide.”