“The first thing to be observed is that it is implicit in the section that this Court may only increase sentences which it concludes were unduly lenient. It cannot, we are confident, have been the intention of Parliament to subject defendants to the risk of having their sentences increased—with all the anxiety that that naturally gives rise to—merely because in the opinion of this Court the sentence was less than this Court would have imposed. A sentence is unduly lenient, we would hold, where it falls outside the range of sentences which the judge, applying his mind to all the relevant factors, could reasonably consider appropriate…………. However it must always be remembered that sentencing is an art rather than a science; that the trial judge is particularly well placed to assess the weight to be given to various competing considerations; and that leniency is not in itself a vice. That mercy should season justice is a proposition as soundly based in law as it is in literature. The second thing to be observed about the section is that, even where it considers that the sentence was unduly lenient, this Court has a discretion as to whether to exercise its powers.”