“I deduce from the authorities which have been cited that, following the trial of a preliminary issue, the court may make an order for costs in favour of the party that has won that issue. Before doing so, however, the court must consider all the circumstances of the case. If the judge is told that the unsuccessful party on that issue has made a payment into court, or a Part 36 offer, the normal order should be to reserve costs. Nevertheless, in an exceptional case, despite such payment in or offer, the judge may still make an immediate order for costs if the circumstances warrant such a course.”