The barrister's conduct must be judged by reference to the material before him/her and any further material or information which the barrister ought to have obtained. The court trying the professional negligence action must have regard to the state of the law when the barrister was acting or advising, not the state of the law at the date of trial.
Per Jackson LJ in Chinnock v Wasbrough & Anor  EWCA Civ 441 at para 52:
Let me turn, therefore, to the law governing a doctor's duty to provide information and advice. The starting point is Bolam v Friern Hospital Management Committee  1 WLR 582. This established that a medical practitioner was to be judged by the standards of his profession. If he acts in accordance with the practice of a responsible body of practitioners, then he is not negligent even if others would have taken a different and more efficacious course. The Bolam principle is, however, subject to a well recognised exception. If the practice of the profession or part of the profession is shown to be unreasonable, then compliance with such practice will not absolve the doctor from liability: see Bolitho v City and Hackney Health Authority  AC 232.