“We must be very careful not to interfere with the judge’s exercise of the discretion which has been entrusted to him. We can only do so if he has erred in law or in principle, or if he has taken into account some matter which he should not have taken into account or has left out of account some matter which he should have taken into account; or - and this is an extension of the law which is now I think well recognised - if the Court of Appeal is of opinion that his decision is plainly wrong and therefore must have been reached by a faulty assessment of the weights of the different factors which he has had to take into account. There are various ways of putting that, and they may not be a very good one; but it is a factor that is a category of error which permits, and indeed requires, this court to reverse or alter the judge’s exercise of his discretion.”