“In essence, on an application of this sort, the court is faced with a balancing exercise between, on the one hand, the obvious desirability of retaining a fixed trial date (which promotes certainty) and avoiding any adjournment (which can only add to the costs of the proceedings) and, on the other, the risk of irredeemable prejudice to one party if the case goes ahead in circumstances where that party has not had proper or reasonable time to prepare its case”