I wanted to see the words. It was surprisingly difficult to find them, but eventually:
https://drive.google.com/file/d/0B5Ik-g ... NJZ1E/view
The relevant bit is Chapter 4 (Page 14)
2. Election of leader and deputy leader
A. The leader and deputy leader shall be elected
separately in accordance with rule C below,
unless rule E below applies.
i. In the case of a vacancy for leader or deputy
leader, each nomination must be supported
by 15 per cent of the Commons members of
the PLP. Nominations not attaining this
threshold shall be null and void.
ii. Where there is no vacancy, nominations may
be sought by potential challengers each year
prior to the annual session of Party
conference. In this case any nomination must
be supported by 20 per cent of the Commons
members of the PLP. Nominations not
attaining this threshold shall be null and void.
The rules seem to make a clear distinction between situations where there is a vacancy, and where there is a challenge. The one states what is required for a nomination, the other does not, but deals only with nominations for potential challengers.
http://www.labour.org.uk/page/-/blog%20 ... ines_2.pdf
It seems further that the National Executive retains some discretion to interpret the rules and to resolve differences. I cannot see that a legal challenge would succeed.
More than that, This will put a very clear question to the NEC - to which side will they give their support? I have to say that I cannot see that it would be right to require Corbyn to find nominations.
That said, the apparent importance of the question only emphasises the fatally deep split within the party.