“It is of course not enough for a tribunal to be in factual error; far from it, it is required before this tribunal can interfere, that there be an error of law. There can in rare circumstances be such a concatenation of errors of fact, or so gigantic a howler, that such error might amount to perversity, but perversity, as has been made clear in so many recent authorities in the Court of Appeal, is a very narrow ground.”