When Labour’s ruling national executive committee (NEC) met on Tuesday, Jon Lansman — the gnome-like founder of Momentum, the grassroots supporters of Jeremy Corbyn — mischievously sat down next to Jennie Formby, the political director of Unite, Britain’s biggest union.

A month ago this would have been a scene of hard-left harmony, a duo representing Corbyn’s loudest cheerleaders and his party’s biggest paymaster. Last week it was an act of passive aggression as the two wings of the Corbynista coalition found themselves locked in a bitter civil war to control the party.

Formby, who is Corbyn’s candidate to succeed the moderate Iain McNicol as Labour’s general secretary, represents the power of organised labour. Lansman is her challenger, a man who has been on a mission