“If, in this kind of action, it is plain that an action could have been brought, and if it had been brought that it must have succeeded, of course the answer is easy. The damaged Plaintiff then would recover the full amount of the damages lost by the failure to bring the action originally. On the other hand, if it be made clear that the plaintiff never had a cause of action, that there was no case which the Plaintiff could reasonably ever have formulated then it is equally plain that the answer is that she can get nothing save nominal damages for the solicitors’ negligence…The present case, however, falls into neither one nor the other of the categories which I have mentioned. there may be cases where it would quite impossible to try ‘the action within the action’… In my judgment, assuming that the plaintiff has established negligence, what the court has to do in such a case as the present is to determine what the Plaintiff has lost by negligence. The question is, has the plaintiff lost some right of value, some chose in action of reality and substance? In such a case it may be that its value is not easy to determine, but it is the duty of the court to determine the value as best it can.”
“20. For present purposes the courts have developed a clear and commonsense dividing line between those matters which the client must prove, and those which may better be assessed upon the basis of the evaluation of a lost chance. To the extent (if at all) that the question whether the client would have been better off depends upon what the client would have done upon receipt of competent advice, this must be proved by the claimant upon the balance of probabilities. To the extent that the supposed beneficial outcome depends upon what others would have done, this depends upon a loss of chance evaluation. 21. This sensible, fair and practicable dividing line was laid down by the Court of Appeal in Allied Maples Group Ltd v Simmons & Simmons[1995] 1 WLR 1602 … 22. The Allied Maples case was about the loss, due to negligence, of the opportunity to achieve a more favourable outcome in a negotiated transaction, rather than about the loss of an opportunity to institute a legal claim. But there is no sensible basis in principle for distinguishing between the two, and none was suggested in argument. In both cases the taking of some positive step by the client, once in receipt of competent advice, is an essential (although not necessarily sufficient) element in the chain of causation. In both cases the client will be best placed to assist the court with the question whether he would have taken the requisite initiating steps. He will not by the defendant's breach of duty be unfairly inhibited in proving ata trial against his advisor that he would have done so, save perhaps where there is an unusual combination of passage of time and scarcity of other probative material, beyond his own unaided recollection. 23. Two important consequences flow from the application of this balance of probabilities test to the question what the client would have done, in receipt of competent advice. The first is that it gives rise to an all or nothing outcome, in the usual way. If he proves upon the narrowest balance that he would have brought the relevant claim within time, the client suffers no discount in the value of the claim by reason of the substantial possibility that he might not have done so: see Stuart-Smith LJ in the Allied Maplescase[1995] 1 WLR 1602 , 1610GH. By the same token, if he fails, however narrowly, to prove that he would have taken the requisite initiating action, the client gets nothing on account of the less than 50% chance that he might have done so. 24. The second consequence flows directly from the first. Since success or failure in proving on the balance of probabilities that he would have taken the necessary initiating step is of such fundamental importance to the client's claim against his advisor, there is no reason in principle or in justice why either party to the negligence proceedings should be deprived of the full benefit of an adversarial trial of that issue. If it can be fairly tried (which this principle assumes) then it must be properly tried. And if (as in this case) the answer to the question whether the client would, properly advised, have taken the requisite initiating step may be illuminated by reference to facts which, if disputed, would have fallen to be investigated in the underlying claim, this cannot of itself be a good reason not to subject them to the forensic rigour of a trial. As will appear, this has an important bearing on the extent of the general rule that, for the purpose of evaluating the loss of a chance, the court does not undertake a trial within a trial.”
“In order to recover for the loss of that kind the court must be satisfied that the plaintiff had at least a "real" or "substantial" chance that he would have succeeded in the primary action, not merely a speculative one: see AlliedMaples Group v Simmons & Simmons [1995] 1 W.L.R. 1602 per Stuart-Smith L.J. at 1614. If his prospects of success fall short of that, the court will ascribe no value to them, but provided the court can see that there were real prospects of success it will evaluate them notwithstanding the difficulties that may involve. The need to evaluate the prospects of success in that way usually arises because of uncertainty as to the final shape of the evidence which would have been before the court trying the primary action. In some cases, however, the outcome of the primary action is not in doubt, for example, if it can be seen that the claim is bad in law and could never have succeeded. In such a case, of course, there never were any prospects of success at all.”