POGS in regard to your post @20:03 today, as i have already stated in this thread Momentum and the Trade Union involvement in the Parliamentary Labour Party is set in two arms of the same of the instance. Momentum was born out of the Unite Unions new Comunity Branches with were formed in 2010, while the trades unions have always held a presence in the Parliamentary Party by way of their industrial Branches.
In short, one arm has an industrial base founded in the presence of workplace activists, while the other arm is based in activists from the communities that surround industrial complexes and large trading estates.
Jon Lansman and Jeremy Corbyn became interested in the community based sector of the above and that attracted media attention to the grouping but in essence, they are both still part of the trades unions and the broader Labour movements presence in the Parliamentary Party, which on the industrial side has been over a very long period of time.
There has been a rise in trade union membership especially in the private sector due in all probability to their success in the courts against employers using Gig Economy terms of employment. That has been offset to some extent by a loss of membership in the public sector due to the councils reducing the size of their workforces due to austerity.
However, the above has given a great deal of new confidence to the Trade Union Movement which in part i feel had brought about the questioning of the relationship with the Parliamentary Party following this election and in some quarters the demand that the "plug should be pulled" on that relationship.
In regards to who will decide the outcome of the Leadership Election and the relationship between the Unions and the Parliamentary Party, that will be decided by the whole Labour movement with those who pay the political Levy in the Trades Unions being the largest single block.