In regard to the formation of Momentum and its influence on the Labour party, that can easily be tracked back to the Unite unions forward thinking in the formation of its Community Branches several years ago.
The Labour Party was born out of the Trade union movement in the early decades of the last century. That close ongoing relationship was lost following "the winter of discontent" in 1979 and indeed became very strained during the Blair premiership years. However, following the "Banking Crisis" of 2008 the Unite Union took the decision to tightly focus once again on its political strategy, hence the formation of its Community Branches
The above branches under the leadership of Unite lay union reps met with great success and through the longstanding Unite affiliation to the Labour party, nominated community branch members have been able to take up trade union representative positions long set aside for them within the Labour party district, constituency, regional structure.
The independent Community Branches (also joined later by activists from other leading unions) formed an affiliation under which Momentum was brought into being. I certainly feel that the above has been a good democratic development in Britain's political body. Once again political discussion has been brought back to those who would otherwise find it difficult to have their views considered in places that have the powers to bring about change by way of a mainstream political party.
Some of Britain's greatest statesman came out of trade union movement, Ernist Bevan, Nye Bevan and Clement Atlee to name but a few.