Who Leaked Labour's Draft Election Manifesto?

Speculation is mounting in Westminster over who leaked Labour’s draft election manifesto. 
A host of sources from different factions of the party have been suggested as the culprits, while journalists scramble to uncover how the top secret document was released a day before its contents were due to be confirmed at a meeting of Labour big wigs.
HuffPost UK looks at the claims levelled at three groups in Labour and what’s been said about their involvement in the leak:
1. The lead