“The statement of the prosecution that they would offer no evidence at the next hearing was not merely a statement made to the defendant or to his legal representative. It was a made coram judice, in the presence of the judge. It seems to us that whether or not there was prejudice it would bring the administration of justice into disrepute if the Crown Prosecution Service were able to treat this Court as if it were at its beck and call, free to tell it one day that it was no going to prosecute and another day that it was.”