“In the absence of an identifiable error, such as an error of law, or the taking into account of an irrelevant matter, or the failure to take account of a relevant matter, it is only a difference of view as the apportionment of responsibility which exceeds the ambit of reasonable disagreement that warrants the conclusion that the court below has gone wrong. In other words, in the absence of an identifiable error, the appellate court must be satisfied that the apportionment made by the court below was not one which was reasonably open to it.”