“I start from the position that this is in substance an attempt, after the trial is over and in light of the results of the trial and the findings made at it, to re-litigate an interlocutory costs order. No court, whether on an application to set aside at first instance or by way of appeal, is receptive to such an application. It is bound to be a difficult exercise for the court to review the exercise of a discretion made at an early stage where not all the evidence is available or all the facts known or even the issues identified”