In Johnson , Brennan J said (at 373):
The value of the lost cause of action cannot be assessed as though there were a market for doubtful causes of action in damages for personal injury. The value of the lost cause of action is not what a speculator would be prepared to offer the plaintiff as the price of an assignment of the cause of action. The plaintiff's loss being whatever monetary compensation he would have received at the time he would have received it but for his solicitor's negligence, the court must find whether or not he has lost something of value. If he would have failed in the original action, he has lost nothing; if he would have succeeded, he has lost what he would have received at the time he would have received it; if the action would have been compromised, he has lost what he would have been paid in settlement at the time when he would have been paid. Or, if it is doubtful whether or not he would have succeeded in the action and it is not probable that the action would have been compromised, the court assessing the damages must determine as best it can on the balance of probabilities whether the plaintiff would have succeeded (and, if so, to what extent) or failed. In making that determination, the court may need to estimate the extent to which a successful plaintiff's damages would have been reduced because of contributory negligence.
Johnson and Perez were cases where there was no dispute that the plaintiff whose claim was lost by the negligence of the solicitors had lost something of value. However, this is the critical issue in the present case. Nevertheless there is guidance from the High Court as to how a matter such as the present should be approached. Several of the justices cited with approval a well-known passage in the judgment of Lord Evershed MR in Kitchen v Royal Air Force Association [1958] 1 WLR 563 at 574-5; [1958] 2 All ER 241 at 250-1:
If, in this kind of case, it is plain that an action could have been brought, and, that if it had been brought, it must have succeeded, 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 she can get nothing save nominal damages for the solicitors' negligence. I would add, as was conceded by counsel for the plaintiff, that in such a case it is not enough for the plaintiff to say: "Though I had no claim in law, still, I had a nuisance value which I could have so utilised as to extract something from the other side, and they would have had to pay something to me in order to persuade me to go away".
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 be quite impossible to try "the action within the action': as counsel for the second defendants asks. It may be that for one reason or another the action for negligence is not brought until, say, twenty years after the event, and in the process of time the material witnesses, or many of them, may have died or become quite out of reach for the purpose of being called to give evidence. 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 that value as best it can.