What is the test for estoppel in the context of the doctrine of res judicata?

Diplock L.J. in Mills v. Cooper, [1967] 2 All E.R. 100, expressed the doctrine of res judicata, sometimes known as issue estoppel, in these terms (at pp. 104-105): This doctrine, so far as it affects civil proceedings, may be stated thus: a party to civil proceedings is not entitled to make, as against the other party, an assertion, whether of fact or of the legal consequences of facts, the correctness of which is an essential element in his cause of action or defence, if the same assertion was an essential element in his previous cause of action or defence in previous civil proceedings between the same parties or their predecessors in title and was found by a court of competent jurisdiction in such previous civil proceedings to be incorrect, unless further material which is relevant to the correctness or incorrectness of the assertion and could not by reasonable diligence have been adduced by that party in the previous proceedings has since become available to him. Whatever may be said of other rules of law to which the label of "estoppel" is attached, "issue estoppel" is not a rule of evidence. True, subject to the qualification which I have stated, it has the effect of preventing the party "estopped" from calling evidence to show that the assertion which is the subject of the "issue estoppel" is incorrect, but this is because the existence of the "issue estoppel" results in there being no issue in the subsequent civil proceedings to which such evidence would be relevant. Issue estoppel is a particular application of the general rule of public policy that there should be finality in litigation.