“In considering whether damage was suffered in 1978 one can test the matter by considering what would have happened if in, say, 1980 the plaintiff had learned of his solicitors' default and brought an action for damages. Of course, he would have taken steps to remedy the default. But he would have been entitled at least to recover from the defendants the cost incurred in going to other solicitors for advice on what should be done and for their assistance in lodging the appropriate caution. The cost would have been modest, but not negligible.”