Callistemon Quote [Grandad perhaps the Labour party left him, rather than he left the party. It has happened to so many other people, unfortunately.]End Quote.
Callistemon, there is I feel much truth in what you state above. What many people do not realise is that the Labour Party of today is very different in its organisational structure from the time when Tony Blair headed the then all powerful Parliamentary Labour party, with the broader Labour movement being totally ignored and forgotten.
In the above, it was the lay activists within that broader Labour movement that resolved following the loss of the General Election in 2010 that their primary efforts would be placed into bringing the Parliamentary Labour Party back into the broader movement and answerable to it. Many of those lay activists were (and still are) trade union Branch Secretaries, chairman and workplace union or safety reps with branch funding at their disposal.
In places, those persons directly stepped up their activities within the constituency, regional and district Labour parties, while in other areas they became the first roots nationally of Momentum by way of the Unite Union Community Branches. Their efforts took a number of years to bring about real change, but in 2015 following the election of Jeremy Corbyn he along with others brought forward rule changes that allowed the Broader Labour movement much better representation within the Labour party. That change gave those lay activists Elected Seats on the National Executive Committee and far greater representation at the Annual Party Conference.
I have often met trade union activists such as the above while tutoring on workplace Safety Courses at Unite and GMB offices around the Country. They are people who fight battles every week while representing members in workplaces nationally, and in that very often losing, but always picking themselves up and moving on to the next fight. Those people have no time for career politicians such as the likes of Chuka Umunna and other "moaners" in the Parliamentary Party.
In their eyes of all the above, the changes in the movement have been democratically voted through and should be accepted by all. Those who do not like those changes can if they wish constitutionally fight their corner from within the movement/party, but just "standing on the sidelines moaning" will never be acceptable to them, and it is their opinions that matter in the Labour Party of today.
So, when the Labour Party constitutionally and structurally changed Chuka TheMoaner would not consider changing with it, or remaining to fight those changes. That turned out to be poor judgment on his part. However, he now seems to have made a habit of making "poor judgments" since resigning from the Labour Movement.