"In this situation we would prefer to say that there will be no estoppel unless it appears that the matter relied upon as a defence in the second action was so relevant to the subject matter of the first action that it would have been unreasonable not to rely on it. Generally speaking, it would be unreasonable not to plead a defence if, having regard to the nature of the plaintiff's, and its subject matter it would be expected that the defendant would raise the defence and thereby enable the relevant issues to be determined in the one proceeding. In this respect, we need to recall that there are a variety of circumstances, some referred to in the earlier cases, why a party may justifiably refrain from litigating an issue in one proceeding it wished to litigate the issue in other proceedings e.g. expense, importance of the particular issue, motives extraneous to the actual litigation, to mention but a few.