“Parliament is presumed not to have intended to alter the law applicable to past events and transactions in a manner which is unfair to those concerned in them, unless a contrary intention appears. It is not simply a question of classifying an enactment as retrospective or not retrospective. Rather it may well be matter of degree the greater the unfairness, the more it is to be expected that Parliament will make it clear if that is intended.”
“Precisely how the single question of fairness will be answered will depend on the interaction of several factors, each of them capable of varying from case to case. Thus the degree to which the statute has retrospective effect is not a constant. Nor is the value of the rights which the statute affects, or the extent to which that value is diminished or extinguished by the retrospective effect of the statute. Again, the unfairness of adversely affecting the rights, and hence the degree of unlikelihood that this is what Parliament intended, will vary from case to case. So also will the clarity of the language used by Parliament, and the light shed on it by consideration of the circumstances in which the legislation was enacted. All these factors must be weighed together to provide a direct answer to the question whether the consequences of reading the statute with the suggested degree of retrospectivity are so unfair that the words used by Parliament cannot have been intended to mean what they might appear to say.”