Clause 7:

1 Party officers

1A Leader and deputy leader
(a) There shall be a leader and deputy leader of the party who shall, ex-officio, be leader and deputy leader of the PLP.

(b) The leader and deputy leader of the party shall be elected or re-elected from among Commons members of the PLP in accordance with procedural rule 4B.2, at a party conference convened in accordance with clause VI of these rules. In respect to the election of the leader and deputy leader, the standing orders of the PLP shall always automatically be brought into line with these rules.

This wouldn’t necessarily, from my reading, preclude a ballot taken at a meeting of the PLP, although the ballot would not be binding.  There is no provision for such a ballot.  Since the Party Conference is the body which determines the leadership, it would seem that any ballot would seriously damage Brown and almost certainly trigger a conference.

Interestingly, at 14.25 today, the Labour Party issued a statement which made clear that the leadership is not only open to members of Parliament.  The leader becomes ex-officio leader of the PLP, however.  That would appear to open the door to Mandelson.