The Labour Party is an "unincorporated association" and, as such, the party does not have a separate legal personality from that of its individual members. On joining the Party, individuals agree to the "Terms and Conditions" which include agreeing to "accept and conform to the constitution, programme, principles and policy of the party." The party has its own Rule Book setting out matters such as the party's constitution and detailing the role of the National Executive Committee (NEC). The rules make the NEC - (subject only to Party Conference) - the final authority on the meaning and effect of party rules. There is nothing either unusual or untoward about any of these arrangements. The party constitution is contained in its rules and it is a contract to which each member adheres when he joins the party.
In June 2016, the Parliamentary Labour Party held a vote of no confidence in the party leader. The motion was passed by 172 votes to 40. The eventual outcome of this was that the party began the process of electing a leader and a challenge was taken to the High Court on the question of whether Corbyn, as existing party leader, was automatically entitled to be included in the ballot. The High Court ruled that he was was so entitled - Michael Foster v Iain McNicol and Jeremy Corbyn  EWHC 1966 (QB) Foskett J. This judgment contains some discussion of Clause 1.X.5 of the rules and whether it was an attempt to oust the jurisdiction of the court. I won't dwell on that here but it is worth reading (Para 55 to 59).
Naturally enough, the party continually seeks new members and the question arose as to whether new members were entitled to vote in the leadership election. The NEC decided that an individual had to have been a member continuously for at least 6 months in order to vote in the leadership election. The 6 months ran from 12th January 2016. Some new members considered this to be a breach of contract and took action in the High Court. The case was heard by Hickinbottom J - Evangelou and others v McNicol ]2016] EWHC 2058 (QB) - and the judge found that not allowing the claimants to vote would be a breach of contract. The case went to the Court of Appeal (Beatson, Macur, Sales LJJ) - Evangelou and others v McNicol  EWCA Civ 817 where it was held unanimously that the party could exclude the claimants from voting. The court interpreted the party rules as giving the NEC power to define eligibility for voting and that included setting a retrospective date (freeze date) as it had done here. The law of contract and unincorporated associations is helpfully summarised at paragraphs 18 to 24.
Given the political nature of the subject, it is hardly surprising that the court proceedings have been highly controversial in some quarters but certain comments have been unworthy and risible. One claim was that an appeal court judge (Sales LJ) was not impartial because, as a barrister, he had worked for the government as First Treasury Counsel ("Treasury Devil"). At the court hearing no objection was taken to Sales LJ sitting and, I feel sure, was not even considered.
It should be noted that the courts were not considering a political question even though the answer to it will have some political consequences. The court was concerned with interpretation of the party rules and they had to do that because it was central to the action (based on contract) brought by the claimants. Once a contract is before the courts, it becomes a question of law as to the meaning of the contract - see paragraphs 18 to 24 of the Court of Appeal judgment.
The Court of Appeal held that the party rules permitted the NEC's decision. It follows that those who are disappointed at not being able to vote must point their fingers at the NEC and not the court.
The following tweets by Greg Callus QC may be of interest:
Biographical note:Excellent. Do take time to read it all. https://t.co/cwSenCKbU0— Marilyn Stowe (@marilynstowe) August 15, 2016
Lord Justice Sales was called to the Bar, Lincoln’s Inn in 1985. Barrister in 1986–2008. First Treasury Junior Counsel (Common Law) in 1997–2006. Appointed Assistant Recorder in 1999–2001; Recorder in 2001–08; Deputy High Court Judge in 2004–08. He became Queen’s Counsel in 2006. First Treasury Counsel (Common Law), 2006–08. Member of the Panel of Chairmen, Competition Appeal Tribunal, 2008–; Deputy Chairman of the Boundary Commission for England, 2009 to date; Vice-President of the Investigatory Powers Tribunal, 2014-. He was appointed a Judge of the High Court, Chancery Division in 2008.