“As it seems to us, if this discretion may be exercised in such a way as to refuse to make any allowance for the time spent in custody abroad pending extradition — and plainly the statutory language underlines that it can — it would fall to be exercised where a defendant deliberately fled this country in a well-organised, sophisticated plan to evade justice here; successfully evaded justice for some time by staying abroad; when eventually brought before the courts abroad with a view to extradition, contested the extradition proceedings every inch of the way, and, what is more, put up a totally false story in order to evade extradition followed by...an unsuccessful appeal against the order.”