"That practice may on its face seem harsh. On the other hand, the consequences of any rule are equally unattractive. It would mean that a defendant who had roundly and on advice accepted that he had acted dishonestly and fraudulently, and pleaded guilty, or who had been found guilty and chosen not to appeal, could after the event seek to reopen the convictions. If such convictions were to be readily reopened it would be difficult to know where to draw the line or how far to go back . . . It is plain, as we read the authorities, that there is no inflexible rule on this subject, but the general practice is plainly one which sets its face against the reopening of convictions recorded in such circumstances. Counsel submits - - and in our judgment submits correctly - - that the practice of the Court has in the past, in this and comparable situations, been to eschew undue technicality and ask whether any substantial injustice has been done."