Ultimately I think there's a limit to the degree of ideological unity that Labour (or for that matter the Conservatives) can achieve. In a First Past the Post system, even one that's fraying at the edges as the British one seems to be, there is a tremendous incentive for people with diverse views to converge in large, ideologically loose parties. Political scientists often call these "disguised coalitions" and they often display the kind of ideological messiness that Labour (and the Conservatives) have displayed.
This kind of disunity isn't necessarily fatal - the Conservatives have had their share of internal arguments, and it obviously didn't prevent them from exceeding all but their most sunnily optimistic goals electorally. Even Blair's impressive marshalling of the Labour party behind a common banner was only briefly successful - cracks were already showing by 2002, and the Iraq War blew it wide open. I don't think disunity has actually been a problem for the Labour Party recently - I'm struggling to think of an instance where a significant Labour figure went "off message", although it's possible I missed it.
Of course there is a risk of a big internal fight in Labour in the near future, but if I was a Labour strategist I would recommend getting it out in the open right now - have the big messy fight soon after you've lost an election, when the public eye is elsewhere. Better to air your dirty laundry in late 2015 than early 2020.