"Re-opening contentious matters or permitting one or more of the parties to add to their case or make a new case should rarely be allowed. Any attempt to do this is likely to receive summary rejection in most cases. It will only very rarely be appropriate for parties to attempt to do so. This necessarily means that the court would only be prepared to do so in 'exceptional circumstances', but that expression by itself is no more than a relatively uninformative label. It is not profitable to debate what it means in isolation from the facts of a particular case."
"The question whether to exercise the jurisdiction can only depend upon the circumstances of the particular case."
"It is settled that an estoppel by convention may arise where parties to a transaction act on an assumed state of facts or law, the assumption being either shared by them both or made by one and acquiesced in by the other. The effect of an estoppel by convention is to preclude a party from denying the assumed facts or law if it would be unjust to allow him to go back on the assumption ... It is not enough that each of the two parties act on an assumption not communicated to the other. It was rightly accepted by counsel for both parties that a concluded agreement is not required for an estoppel by convention."