A Falkirk coda: who leaked?

One last matter on Falkirk, which did not get raised in yesterday’s piece yet intrigues me, is the following riddle it leaves behind. 

Who was close enough to Miliband to see the full Falkirk report; disloyal enough to leak something damaging to Labour; and who was far-left enough to have thought of Seumas Milne as the ideal vessel, rather than a more serious mainstream lobby journalist, for the leak?

I have a strong suspicion who this might be, but it is only that.