“An accused person is undoubtedly entitled to reserve his defence until the appropriate stage of his trial, but if he intends to call witnesses he must take timely steps to have them at his trial ready to give evidence at that stage-which means that he must summon them in advance, and that he cannot complain if, seeing that he has not summoned them, the trial Court refuses to grant him an adjournment for the purpose of summoning witnesses when he is called upon for his defence.”