"the EAT here was exercising the classic discretion of the trial judge in that case on the issue of witness summonses and like matters. These decisions are entrusted to the discretion of the court at first instance. Appellate courts must recognise that in such decisions different courts may disagree without either being wrong, far less having made a mistake in law. Such decisions are, essentially, challengeable only on what loosely may be called Wednesbury grounds, when the court at first instance exercised the discretion under a mistake of law, or disregard of principle, or under a misapprehension as to the facts, where they took into account irrelevant matters or failed to take into account relevant matters, or where the conclusion reached was 'outside the generous ambit within which a reasonable disagreement is possible."