In regard to how much power Kier Starmer will have in truly reshaping the face of the Parliamentary Labour Party will be decided in the coming weeks when fresh elections for the National Executive Committee (NEC) are held.
For the left to retain its strong influence on that governing committee there will have to be an agreement in regard to a "slate" of candidates between Momentum and the trade unions. Those candidates would go forward without opposition from any other sector on the left of the Labour movement in that ballot.
By example to the above, there are positions on the NEC held by Momentum by way of the Constituency Labour Parties. However, the trade unions also have their own delegates within those Constituency Labour Parties also which could stand against the Momentum candidates. Such a battle however would allow right wing candidates "through the middle" which would be a disaster for the left on the NEC.
There are several reasons why the leading trade unions may not agree to such a slate of candidates being drawn up and agreed for this NEC election. However, in whether such an agreement is struck may well decide how much freedom Starmer and his new shadow cabinet will have in reshaping the Parliamentary Labour Party in the coming months.
For what it is worth, I do not feel that such a slate will be agreed this time around.